If you believe the right-wing press, Peter Tatchell has gone through an amazing transformation from villain to hero in the past decade. In 1995 he was 'Public Enemy No 1' according to the Sunday Times and 'Prize Pervert' in the Express. But by 2001 he was 'a national hero' to the Sunday Times, and 'an example to us all' in the Mail. So what happened? Has he really changed so much? Or has the public finally got used to him?

Tatchell is 52. He originally came to England (from Australia) in 1971 to avoid the Vietnam draft, took a degree in social sciences and stood unsuccessfully as a Labour MP in 1983. He founded OutRage! with a group of friends in 1990 to campaign for gay rights. Their first campaigns were rather frothy - they held kiss-ins at Piccadilly Circus, and Queer Christmas Shopping Extravaganzas with lots of men dressed as fairies. But they also ran a serious and successful campaign against police harassment, by putting up photographs everywhere of the 'pretty police' who were used as bait in public lavatories. Within three years, the number of gay men convicted of gross indecency fell by two-thirds. At this point, Tatchell was certainly a hero to the gay community, if not to the wider world.

But then, in the mid-Nineties, he turned his guns on the Church of England and things began to get ugly. He disrupted the enthronement of the Bishop of Durham and the Archbishop of Canterbury's Easter sermon. He also 'outed' 10 gay bishops, forcing one of them, the Bishop of London, into an unhappy admission that his sexuality was 'a grey area'. Tatchell followed this with a threat to out 20 gay MPs who had expressed anti-gay views. He didn't do so in the end because, he says, the threat of exposure was so effective that they quickly readjusted their ideas. But many people who had hitherto admired Tatchell's work were turned off by what they saw as blackmail and bullying tactics.

Then, in October 1999, he attempted to perform a citizen's arrest on President Mugabe during his state visit to London. Tatchell recalls: 'We ambushed his motorcade as he was leaving his hotel. I grabbed Mugabe, and told him he was under arrest on charges of torture. You should have seen the look on his face! Then the police arrived and, even though I had all the legal paperwork, we were all arrested and President Mugabe was given a police escort to go shopping at Harrods.' In 2001 Tatchell attempted a similar arrest in Brussels and was beaten so badly by Mugabe's thugs that he now suffers lasting brain damage. It was at this point that the right-wing press started hailing him as a hero - everyone admired his courage and even the Telegraph called him 'a national treasure'.

So which is he? Hero or villain? Selfless human rights campaigner, or hysterical self-publicising fanatic? When I phone to suggest a meeting, he says I can interview him on the train to the Green Conference in Weston-super-Mare, where he is giving a speech. There are several things wrong with this plan - first it entails going to Weston-super-Mare, second it entails attending the Green Conference, third it entails doing an interview on a train. But instead of screaming, 'No!' as all my instincts tell me to, I meekly say yes. Peter Tatchell has this way of quietly co-opting you into his world.

He is easy to recognise at the station - chiselled face, vegan shoes. But it is hard to find seats on the train because it is packed full of Greens. I thought we'd done well, bagging a table for four, but soon an elderly man comes and sits next to us, and opens his newspaper - The Church Times. I find this extremely inhibiting, but Tatchell is unfazed. He is not ashamed of his views, so why should he mind trumpeting them (he has a very loud voice) to anyone within earshot? The way he talks is strange. It is as if he has files of prepared speeches in his brain; and when you ask him a question he recites the speech at dictation speed. But sometimes he loses his thread and then he goes right back to the beginning. You can find all the speeches on his website - www.petertatchell.net - so I won't bother to reproduce them here. They cover subjects from 'Arm the Kurds' to 'Why animal research is poor science' to 'Rename Aussie capitals', as well as all the more predictable gay topics.

He apologises for his occasional lapses of memory and blames them on the 'Mugabe factor'. 'The specialist told me that in cases of severe concussion, one of the side effects is quite often memory and concentration impairment. He hoped it would be temporary - and it isn't as bad as it was, but it's still far from perfect. It's also affected the vision in my right eye - it is now blurred, though before I had perfect vision.' The Mugabe beating was probably 'one of the worst' he's received, but he is used to being attacked. 'I have had bricks and bottles through my windows, three arson attempts, I've been physically assaulted in the street hundreds of times in the past 20 years - often just going shopping.' So why does he take all these risks? Does he want to be a martyr? 'No! I don't want to be a martyr because martyrs end up dead. I want and intend to live a long life. But it's a very small risk compared to what most human rights campaigners around the world endure. If I was campaigning in Iran, I'd probably be stoned to death.'

He is still pursuing Mugabe - earlier this year he applied unsuccessfully to Bow Street Magistrates Court for a warrant for his arrest and extradition. But he and his OutRage! colleagues have many other campaigns on the go. The one that has attracted most recent attention is against what he calls 'murder music' - Jamaican queer-bashing reggae lyrics. He and fellow campaigners have managed to get dozens of concerts and record contracts cancelled, and have persuaded companies such as Pepsi Cola and Cable & Wireless to withhold sponsorship. When I argue this constitutes artistic censorship, Tatchell starts reciting case histories of Jamaican 'batty boys' who've been beaten up and even killed in the street while the police stand by and watch. He says, 'When you hear horrific stories like that you feel a sense of imperative to do something.' 'Well you do,' I tell him, 'I don't.' 'But how can anyone not feel concerned about that? Aren't we all part of the same human family?'

I don't know why the expression 'human family' always fills me with such alarm, but it does. I subscribe to Auberon Waugh's dictum that if you want to increase the sum of human happiness in the world, then the best thing most of us can do is just be happy, rather than wringing our hands every time we open the Guardian. Tatchell, of course, disagrees: 'How can anyone be happy knowing that hundreds of millions of people on this planet are suffering terrible injustices? How can you just turn away and say, "I'm sorry, it's not my responsibility."?'

'Well, lots of people do, including me.'

'But that's the reason why suffering persists - because good, decent people don't take a stand.'

Anyway, it is hopeless trying to argue with Tatchell on the train - he just goes on reciting his speeches. In the Nineties, most of his campaigns were about gay discrimination in Britain, but now he has gone international and taken up causes all over the world. To me, there seems something promiscuous about his geographical range: it looks as though he is looking for battles to fight. Also, it annoyed me that, when he was talking about an old campaign to make Russell Square in London a police-free gay cruising ground, he said it was stymied by 'local busybodies'. It turned out what he meant by busybodies was people who actually lived round the square and objected to the goings-on at night. But if they are busybodies, what is he? He's never even been to Jamaica or Zimbabwe. As the train rumbles westwards, many Greens come and greet Tatchell and tell him to keep up the good work. Tatchell joined the Green Party earlier this year, having resigned from Labour in 2000, and says he likes it because, 'It's all sincere, no spin, it's all grass roots, spontaneous and absolutely genuine. The policies come out of people's needs and concerns without imposing a dogma or ideology on a situation. I like that, because, having been brought up in a very strictly religious family, I've always been very distrustful of any kind of dogmatic ideology.'

(Yeah - right. Except I run up against dogmatic ideology the minute I step off the train at Weston- super-Mare and light up a much-needed cig. Half a dozen Greens immediately turn round and tell me - caringly - 'I'd rather you didn't.' I want to scream at them, 'Well I'd rather you didn't grow hair in peculiar places and talk in whiny voices,' but I sullenly stamp out my cig, whereat they all stare at it lying on the pavement as if I have just vandalised the Mona Lisa.)

The Green Conference in the Winter Garden Hotel offers a day of discussion forums with no smoking, no alcohol and no edible food. Even the tea is some sort of ethically correct stuff grown in a country evidently unfamiliar with tea. Tatchell gives his speech about murder music and batty boys, all of which I have heard on the train. He is meant to be giving another speech later, but I can't face it. 'Just going for a short walk', I tell him, and leg it to the station and bat-out-of-hell to London.

By this point I was deeply fed up with Tatchell and his works. I was bored rigid with his speeches and their robotic delivery. I got no sense of what he was like as a person, beyond a vague suspicion that he had certain histrionic Saint Sebastian tendencies. So, although I wrote to apologise for my flight from Weston-super-Mare, and asked if we could meet again, I wasn't exactly looking forward to it. But by chance, I bumped into Marcelle d'Argy Smith, ex-editor of Cosmopolitan, who, rather improbably, is a good friend of Tatchell's. 'He was so boring!' I wailed. 'Oh, but he's so sweet!' she said. 'Take him some chocolate,' she advised. 'He loves chocolate.' So, armed with a stash of Green and Black's, I went to visit him at home.

He lives in a council flat on a fairly grim housing estate near Elephant & Castle, southeast London. All the flats have steel-lined doors and serious locks and peepholes, but his also has a notice saying it is under CCTV surveillance. The flat would be quite spacious if every inch were not covered with stacks of files and piles of papers. Finding somewhere for me to sit involves him re-organising the whole sofa filing system. There are also two bikes in the room and another in the bedroom, plus fax, photocopier and computer. The phone rings constantly because, Tatchell explains, his email is under 'more or less constant cyber-terrorist attack from Christian and Islamic fundamentalists and neo-Nazi groups.' (I must try saying that next time I complain to AOL.) Just to add to the chaos, there are two huge triffid houseplants, which try to throttle me every time I move. But the room, though cluttered, has many sweet home-making touches: a row of postcards on the mantelpiece, beads hanging from the curtain rails and fairy-lights on the triffids. And all round the walls there are lovely collages of postcards and badges he made in his younger days. They are colourful, witty, engaging - but now almost hidden behind the ramparts of files. Is this the story of his life?

The chocolate does its job brilliantly - not that he eats it, but he is genuinely grateful. He offers me some delicious apple cake he made himself, but says he won't have any now because he wants to 'prioritise breakfast'. Huh? It is 2pm, but he says he hasn't had time to eat breakfast because he's been so busy. 'Are you weird about food?' I ask. No, he explains, but he doesn't like to eat and talk because eating is painful for him - his teeth are all chipped from his many beatings-up. I wish he would eat - he is alarmingly thin.

He once wrote an article for the New Statesman in which he described what it is like living on £7,000 a year. (He earns that from journalism; he gets nothing for his campaigning work, and takes no state benefits.) He says he's better off than most people in the developing world because at least he has clean drinking water (actually, he's better off than most British pensioners) but, nevertheless, he has to live frugally. He cycles everywhere, buys his clothes in charity shops (and looks very smart in them), gets most of his food from markets and only occasionally buys a bottle of wine as a treat.

I say I find it odd that he campaigns for sexual freedom when, by nature, he seems to be rather ascetic. 'Not by choice! I see no virtue in asceticism. It's true I don't believe in materialism and consumerism. They dehumanise us and coarsen personal relations and society. But I've got no desire to be wandering round in sackcloth and ashes. I could quite easily enjoy lazing on a beach, doing nothing in particular.' For how long? 'A few hours.'

He works six, often seven days a week, and virtually round the clock, because his campaigns cross time zones around the world. When he goes to bed, he sticks the phone under a cushion and smothers the answering machine. 'I'm driven to work at this frantic pace not by desire but by necessity. If a millionaire gave me the money for an office, and the six full-time staff I need, and a decent salary, I'd accept it tomorrow. I would love to reclaim this small flat as the living space it was.'

I'm not sure I believe him. He is so driven and busy-busy, what would he do if he had more leisure? 'I could very happily spend a lot more time with my close friends, indulge my passion for nightclubbing, hiking and cycling. I'd see some films and plays, put more time into relationships.' He has not had a serious relationship for the past 15 years, though he has just started one with a young Algerian asylum seeker. 'But I would prioritise spending more time with friends. Friends are very important. A lot of gay men prioritise sex, but to me the most important thing is friendship - long, strong, enduring friendship.'

So how did he end up here? What made an Australian window dresser whose childhood pleasures were surfing, hiking and art, end up as a full-time human rights campaigner in Elephant & Castle? On the train, asking about his past was like pulling teeth, but at home he is more expansive. He grew up in Melbourne, in a working-class family. His father left when he was four, but was replaced by a taxi-driver stepfather who gave his mother three more children and 'ruled with an iron fist'. The family were Pentecostalists - 'very devout, strict, fairly fundamentalist Christians' - who said grace before meals, and had Bible readings in the evening. They disapproved of drinking and smoking, and were pretty suspicious of dancing. Homosexuality was, of course, an unspeakable sin.

He discovered he was homosexual at 17, when he slept with a man and knew immediately: 'This is what I am and it is wonderful.' But he thinks in retrospect he was homosexual all along. 'I remember that my mother took us for a picnic to St Kilda beach when I was eight or nine, and we were sitting on the sand and I can remember looking up and seeing this young man, about 18 or 19, walk by in a very tiny pair of aqua-blue swimming trunks, and I turned to my mother and said, "Could I have some of those?" And she said, "Fine." But what

I think I was really saying in my mind was, "Can I have some of him?" And the image of that man and his physique have stayed with me for years - is still with me now. He wasn't huge, but he was a big, typical blond, bronzed surfie type.'

Something else happened to him when he was eight or nine, which I believe might be the key to his character. His mother started getting very bad asthma attacks, which meant she was often bedridden or in hospital, and he was left to look after his three younger half-siblings. 'So I grew up very quickly, learning how to cook, sew, iron, from age eight or nine. It was like, my mother was sick, my younger sister needed her nappy changed, so I just had to think, "How do I remember Mum doing it? And, of course, the first few times I did it completely wrong, and the nappy kept falling off, but then I got better at it.' Where was his stepfather all this time? 'Out working - he had to try and get the money to pay all the bills. There was no national health service in Australia at that time, so a huge amount of our family income went on medical bills and prescriptions.'

It sounds an incredibly frightening experience - being left, aged eight, in charge of two children and a baby. But that is not how he remembers it: 'It was a chore, but I sort of enjoyed it. I used to bring schoolmates around sometimes to come and help me. Some didn't like it, but some loved it. It was disruptive of my childhood, but it was also quite exciting, and made me feel I was doing something significant and helpful.' On the other hand, it left him with no desire ever to have children himself. 'I feel I've done the parenting bit.'

I wonder if that early experience of feeling 'significant and helpful' is what led to his becoming a human-rights campaigner. He says he didn't intend to become one. When he left school at l6 he wanted to make a career in shop design and had no plans to enter politics. However, he was soon stuffing envelopes and making placards for various campaigns: anti-Vietnam, anti-death penalty, pro-Aborigine land rights - and, 'The more experience I got, the more useful I became. And one success inspired me to try for another. But in those days [circa 1970] Australia was gripped by a McCarthyite witch-hunt. Anyone with liberal or humanitarian views was denounced as a communist. I was going to be sacked from my first job because my boss saw me on a march against the Vietnam war.' So he came to England in 1971 and has stayed ever since.

What drives him? I believe it is that need to feel 'significant' by helping other people. Derek Jarman once called him 'a hen with her chicks', and there is something quasi-maternal in the way he describes the victims of persecution who come to him for help. Of course he could help other people by stuffing envelopes for organisations like Amnesty, but he likes personal contact, likes being a loner and is not averse to media attention. And he is brave, no question. As he says, there are plenty of people willing to stuff envelopes - but rather fewer prepared to be beaten up by Mugabe's thugs