On moving to London in 1971, after refusing to be drafted into the Australian army to fight in the war in Vietnam, he became a leading activist in the Gay Liberation Front. Since then, he has been prominent in nearly every major struggle for homosexual rights in Britain.

At a stroke, the 'homosexual terrorist' Peter Tatchell has become a civil rights campaigner whom we can all applaud. But how did he find the courage to try to arrest Mugabe? 

When I rang to arrange this interview, Peter Tatchell called back to apologise in advance: "I'm not really myself," he said. "I'm still getting blackouts and I can't really think properly." Then he gave me directions to his South London council flat: "It's in the middle of the block. There isn't a number, for security reasons, but you can recognise it from the black bars on the door and windows." 

When I found it I wondered whether there was anyone in. On a cold, grey afternoon, there were no lights burning; nor would a single, long-life bulb, hanging from the ceiling without benefit of a shade, be switched on until two or three hours later, when it was virtually too dark for me to take notes without it. 

It is not that Peter Tatchell is inhospitable - no sooner had I got through the door than he was taking my coat and offering me a cup of tea - it is just that he cannot afford a moment's unnecessary electricity. He makes only about £6,000 a year from his writing. 

I asked him how he was. "I've got bruises and sprains virtually over my whole body. The doctor's view is that there's no permanent damage, but I've got very severe pain around the side and back of my head. The vision in one eye is blurred and there are moments where I have little blanks in the mind." 

Tatchell's many injuries were the reason I was there. Last Monday this one-time demon of Middle England became its most unlikely hero, thanks to his remarkable protest against Robert Mugabe, President of Zimbabwe, in the lobby of the Brussels Hilton. 

H e describes his emotions in the run-up to the confrontation with characteristic formality, as if everything he says could be taken down and used in evidence against him (as it has been, frequently, by reporters and policemen alike). "Despite having done such protests many times before, I always feel incredibly nervous. My heart is pumping. I feel sick in the stomach. I get incredibly cold. There's a terrible fear that I might not succeed." 

When Mugabe and his entourage appeared in the hotel lobby, Tatchell approached him and said: "President Mugabe, you are under arrest on charges of torture. Torture is a crime under the UN Convention on Torture, 1984. You authorised the torture of Ray Choto and Mark Chavunduka." 

"The last part was pretty muffled," Tatchell remarks, a mischievous smile playing on his lips, as it does throughout his account of events. By then, Mugabe's guards had their hands over his mouth. They punched and kicked him while their leader made his escape and Belgian police and secret service agents looked on. 

Luckily for Tatchell, so many Mugabe henchmen crammed into the Hilton's revolving door that it stuck, trapping the Zimbabwean president. His guards ran towards their master, leaving Tatchell on the floor. He got up, raced round to the front of the hotel and stood directly opposite Mugabe, repeating his allegations. "When he saw me reappear, right in front of him, he had a look of absolute horror on his face. I thought it was divine justice." 

Belgian agents now swung into action, grabbing Tatchell just long enough for two of Mugabe's guards to emerge from the hotel, confront him, and threaten him with death. Then they began hitting him again, stopping only when they saw that the presidential motorcade was about to leave. 

Tatchell picked himself up again. He ran round to Mugabe's car and stood in front of it, shouting out: "Arrest this man, he's a torturer." The police did nothing. The guards got out of the car. He was beaten for a third time and left semi-conscious in the gutter. 

He came to and, undeterred, followed Mugabe to his meeting with the Belgian Prime Minister, Guy Verhofstadt. He stood outside until threatened once again by Zimbabwean bodyguards, who then began filming him "for our records". Tatchell responded by going up to the cameraman and reciting a long list of Mugabe's atrocities. "Anyone shown the film with a view to bumping me off will hear a robust critique of the regime," he smiled. 

He has, he claims, received 300 e-mails from people in Zimbabwe: "What I did apparently brought great joy to millions of Zimbabweans, black and white." That's just the half of it. Tatchell's action embarrassed the European governments who welcomed Mugabe, not to mention the British Government, which has done nothing to combat Mugabe's seizure of white farmers' land and brutal repression of his people. Robin Cook's "ethical" foreign policy is a sham, says Tatchell, who resigned from the Labour Party last year over the "rigging" of the London mayoral selection contest and the "absolute insult" of the 75p rise in pensions. 

But most remarkably, Tatchell's solitary protest (he had tried but failed to find others to join him) has transformed his image in Britain. He is best-known - and least-liked - as the founder of the radical gay pressure-group OutRage. Its attempts to "out" public figures have included the seizure of the Archbishop of Canterbury's pulpit on Easter Sunday 1995. That week, Tatchell was denounced in the press as a "homosexual terrorist" and "prize pervert", yet now he is being praised for his lone stand against dictatorship. For days, The Daily Telegraph letters page bore witness to his new status. 

Tatchell insists that he was not motivated purely by the President's notorious hatred of homosexuals. "It's true that Mugabe has said that gays are 'worse than pigs and dogs' but I've been opposed to him ever since his massacres in Matabeleland in the early Eighties. This was the first time that I had been advised in advance of his travel schedule. He doesn't tour abroad very much. So it was a rare oppurtunity to confront him." 

He grinned at the idea of having another go. "It would be great to have an organisation dedicated to pursuing dictators around the world, harassing, shaming and embarrassing them. At the moment they can swan around the world's capitals being welcomed with open arms." 

Tatchell has lived in London since 1971, but was brought up in Australia and in some poverty. His father left home when he was four, and his mother remarried a fundamentalist Christian, who condemned swearing, drinking and premarital sex. Tatchell later rebelled against the puritanical faith in which he was raised, but admits he is driven by some of that early teaching. "My parents taught me to stand up for what was right, even if it was unpopular or personally difficult. There are more important things in life than career or material wealth. I understand why people feel they need to make compromises, but I find it incredibly difficult. 

"There is a part of me that would love to give up the stress and strain of political struggle and go off to climb volcanoes, surf waves, abseil down waterfalls and laze on the beach. If I could achieve things by having a very low profile and a nicely paid job with no hassle, that would be divine," he adds. 

Yet he has never made the slightest attempt to capitalise on the fame his protests have brought him. He has not, as Derek Hatton did, parlayed his radical image into media celebrity. He has no agent and has never been invited on to television shows such as Question Time or Have I Got News For You. Nor has he been willing, or able, to find one of the many well-paid jobs that exist at the more respectable end of the human rights movement. 

Instead, at 49, he looks and lives like a student. His skinny frame is clad in cheap, monochrome clothes. His feet are shod in frayed black boots which are not made of leather because he is a strict vegetarian. When he ushered me past the tiny kitchenette, I caught a glimpse of the single bedroom where he sleeps on the floor, with a fire-extinguisher by his mattress in case of arson attacks. 

This monastic devotion to the cause (he has had "five great love-affairs" in his life, but relationships are difficult when you live in the way he does) is surely another relic of his religious upbringing. "I love other people and I hate injustice," he says. "I've probably got an exaggerated sense of empathy. If I were a black Zimbabwean, being terrorised by Mugabe's thugs, I would want someone to stand up for me. And that's what keeps me going - the sense that I've helped draw the world's attention to their suffering." 

By now we had been talking for more than two hours. Tatchell's demeanour, which had been cheery at the start of our interview, was noticeably more subdued. From time to time I could see him wince in pain as another headache struck. But he was clearly determined to continue until every possible misconception anyone might have about him had been confronted and despatched. 

For someone who seems to play the media and political game so knowingly, Tatchell is unworldly about their baser realities. Although his opinions seem to be surprisingly moderate, he appears unable to accept that his controversial protests are bound to make sensible discussion of his ideas impossible. 

Instead, he says: "I am strong enough, or crazy enough, to realise that if I keep slogging away, one day the truth will out and people will realise that a lot of what's said about me is nonsense." 

Maybe that day has come. In the dictatorial style of the Mugabe regime he has finally hit upon a cause that the rest of us care about.