Peter Tatchell is not very well. He is still suffering from head injuries sustained when he was attacked by neo-Nazis during a Gay Pride march in Moscow in May. His right eye is damaged, the lining of his brain is swollen, and he feels like he is "drowning, mentally drowning". He fumbles for words, and stops in the middle of sentences to clutch his head. And yet he has spent the morning campaigning on an asylum case, and the afternoon being interviewed by me. Why, I wonder, did he not stay in bed? "Oh, go away!" he responds, tetchily.

Persistence is certainly one of Tatchell's main character traits. For 40 years, he has doggedly pursued campaigns on a bewildering range of issues (his website lists over 30 areas of interest, and includes postings on subjects from animal testing to the independence of Somaliland). Over the course of his career he has, in addition to the Moscow attack, been beaten up by Robert Mugabe's bodyguards, threatened by Muslim militants, interrogated by the Stasi, and ridiculed, often in unacceptably hostile and personal terms, by both the press and his political opponents.

Through all these trials he has come out not only fighting, but stronger than ever. Perhaps he has surprised even himself - as Ben Summerskill, the chief executive of Stonewall, says: "The idea that Peter would assume as visible a place in the national consciousness as his compatriot Dame Edna Everage is probably not something either of them would have anticipated." His citizen's arrests of Robert Mugabe in 1999 and 2001 endeared him even to the Daily Mail, and one commentator christened him a "national treasure" (Tatchell objected strenuously). He is not about to let a little brain swelling stop him now.

Neither, however, will he be softened by his newfound social acceptability. Perched on a hard-backed chair in the cramped living room of his flat in Elephant and Castle, south London, he still resembles the ideal-type campaigner for social justice: lean frame, simple, faded clothes, socks and sandals. Washing up overflows from the sink. The walls are covered in posters. Disappointingly, no offer of a meal is forthcoming; one visitor reported being served carrot soup, followed by carrot casserole. I settle for camomile tea.

"I'm grateful there's a more sympathetic public perception of me now," he says, "but I am also a bit resentful and insulted, because I haven't changed." To prove his point, he reels off a list of his concerns of the moment: the deportation of gay asylum-seekers, the hypocrisy of the Anglican church, the government's refusal to arrest clerics who incite violence against gay people. Despite the reforms of the past ten years, he feels that the Blair government's reputation for being progressive on gay issues was ill-deserved. "Labour's commitment to equality is paper-thin." Tatchell will not stop campaigning, it seems, until he has achieved a complete overhaul of, well, everything. "I don't want equality with straight people in a system which is fundamentally unjust. I want a better system for everyone."

What does this mean? Well, for a start, ditching civil partnerships, to which Tatchell is vehemently opposed. "It sets a very dangerous principle of one law for gay people and another for straight people. In a democracy we should all be equal under the law." Instead, he proposes that all relationships of "care and commitment" should be recognised in law. "I find it very disappointing that legal rights are accorded only if the people are having sex. That should not be the only criterion." Everyone, he says, should be able to nominate a significant other, be they male or female, partner or friend.

There is something very un-British about Tatchell's irony-free approach (he was born in Australia but has lived in this country since 1969). He cites as his heroes Mahatma Gandhi, Martin Luther King, Sylvia Pankhurst and Malcolm X, but even the most favourable assessments have presented Tatchell himself as an eccentric, rather than a fearless freedom fighter.

Although he is now a fervent atheist, his distinctly religious zeal can also raise hackles. Tatchell's mother and stepfather were both Evangelical Christians. It was not a happy upbringing - he has referred to his parents as "real loonies" - but the connection with his proselytising style seems obvious. He, naturally, completely denies this. "To me, religion is a form of ignorance and superstition. My campaigning is the opposite of the dogmatism that drives religion. It is based on knowledge, facts, contact with other campaigners, hearing different points of view." Does he ever change his mind? "Yes," he says, hesitating slightly. "Sometimes."

Despite all this, few would deny that Tatchell has an incredible ability to get things done. He elevated protest to a performance art form with campaigns such as "kiss-ins" and the "outing" of gay Anglican bishops during the 1990s. He has put important issues on the agenda and often achieved real results: he points out that even though the outing campaign was widely criticised at the time, it led the Church of England to make its "strongest ever condemnation of homophobic discrimination". His more recent campaign against anti-gay lyrics in dance-hall reggae resulted in some of Jamaica's biggest stars agreeing to renounce homophobia.

Tatchell judges his achievement to be "helping a process of cultural change, which seeps into institutions. Police treatment of gays, for example, is now completely different. It is hard to identify one thing which brought about that change, but I know, because police officers tell me, that my persistent campaigns did have an effect."

These successes have also come at a very real personal cost. Like many maverick figures, Tatchell is intensely sensitive and prone to depression. "I am hyper-self-critical, and frequently consumed by self-doubt," he says. "I do listen, perhaps too much, to my critics. I'm not an ultra-macho, couldn't-care-less kind of person, and that makes me quite emotionally and psychologically vulnerable. The attacks on me from my colleagues on the left and in the gay movement have been particularly difficult." How has he coped? "There have been moments when I have suffered from extreme depression and despair. I have been helped by a small number of friends, and by my ability to step back and take a longer-term view."

Not many people would be prepared to make such sacrifices. The question is whether, now that Tatchell has attained national treasure status, a younger generation of activists will appear to take up the baton. Tatchell is pessimistic. "Sections of the gay movement pitched their agenda around law reform, and now most of that is won the movement has fallen to pieces. This is something myself and [his campaign group] OutRage! warned about years ago."

Others see the situation rather differently. In their view, having used high-profile tactics to establish the principle of lesbian and gay equality firmly in the national consciousness, the British gay and lesbian movement is settling down to the nitty-gritty of engaging with mainstream politics and drawing up legislation. "Social progress has been achieved by a constellation of different activists, of which Peter is a glittering part," says Summerskill, "but you won't find him sitting down to the hard work of drafting and redrafting." Tatchell, meanwhile, has moved on, arguing that the "real front line" in the struggle for gay rights is now in countries such as Uganda, Zimbabwe, Iran, Iraq and India.

There is also a sense, however, that the great social changes that have taken place in Britain will produce a very different kind of gay politics. When I asked one gay friend in his twenties what he thought of Tatchell, he said that, while he respected his achievements, he felt alienated by a generation that "defined itself by its sexuality". Gayness, for him, was just one component of his identity, and not necessarily the most important one. Tatchell takes it as a compliment. "That is an accolade for the achievements that the gay community has collectively won." He says it with a hint of pride.

Gay UK: the milestones

1957 Wolfenden report recommends decriminalisation of private acts between consenting men

1967 Decriminalisation in England and Wales

1970 First gay rights demo, in Highbury Fields, Islington

1980 Scotland decriminalises, followed by Northern Ireland in 1982

1984 Chris Smith is first MP to come out

1990 Lesbians win firm right to embryonic fertilisation treatment

2000 Ban on gays in armed forces lifted

2001 Age of consent lowered to 16 in England and Wales

2003 Section 28 repealed

2005 First UK civil partnerships