The New York Times calls Irshad Manji "Osama bin Laden's worst nightmare", and no wonder. She is a Canadian Muslim, lesbian, feminist television presenter with spiky orange hair and round glasses, and she is in London to promote what she calls "Operation Itjihad", a liberal reformation of Islam.

The venue is the Royal Institute of British Architects. A clinical room, with chocolate-coloured carpets and stainless steel chairs, plays host to a small audience of mostly white, middle-class intellectuals, plus an Anglican priest.

The setting, near the wide avenues of Regent's Park, is as far as one could imagine from the sectarian turmoil raging in the Middle East and Asia.

Even so, the man sitting in front of me turns to his neighbour and mutters, "You saw the attacks on Galloway... ?" Both glance at the door. He means the MP George Galloway, who was threatened by Islamic extremists during last month's election campaign. Clearly, both are worried that Manji could attract similar violence tonight.

At 5.43pm, moderator Keith Didcock, who has organised the lecture for the Foreign Policy Centre, looks at his watch and nods at Manji. She attempts to start on a congenial note by remarking on the heavy London traffic. The audience smiles politely. She turns solemn.

"I acknowledge that every religion has its share of literalists. Catholics, evangelists, ultra-Orthodox Jews, even Buddhists have their fundamentalists," she begins in a rapid-fire, Canadian accent.

"But the difference is that only within Islam is literalism mainstream and worldwide. We are raised to believe that because the Koran came last chronologically it is perfect. Even moderate Muslims believe it is God 3.0 and none shall come after it. This supremacy complex is worrying. It is dangerous because when violence happens we don't know how to debate and dissent with jihadists. That can't be said about moderate Jews and Christians. Jihadists are adroit at quoting the Koran, and the rest of us feel if we question the jihadists we are questioning our own Koran."

Manji, whose parents fled Idi Amin's Uganda in 1972, published The Trouble with Islam Today amid acclaim and controversy last year. It is an "open letter" to the Muslim world about its ill-treatment of women, persistent practice of slavery and anti-Semitism - all taboo subjects. North American liberals, uncomfortable with politicised Islam but afraid to criticise because they don't want to be labelled racist, were grateful. The television host Oprah Winfrey gave Manji a "Chutzpah Award" for her honesty.

Manji wants Muslims to stop blaming the west and Israel for their perceived humiliation and, instead, revive itjihad, the lost tradition of Islamic free-thinking that allowed the medieval Muslim world to leapfrog ahead of Christian Europe in medicine, literature and mathematics.

The Koran should be studied as a divinely inspired historic document, she says, like the Bible and the Torah. That sounds reasonable enough. Then she says Leicester University withdrew an invitation for her to speak because of threats from Islamists. "It was too dangerous, not only to speak on the campus but anywhere in town," she says. She receives death threats regularly - she won't say how many - and has police protection.

Her book has been published in 18 countries, but Manji says she can't get a Saudi publisher to touch it. In the next breath, she says she has been deluged with so many requests from young Arabs that the book has been translated into Arabic and can be downloaded free from her website.

"My translator in Iran withdrew from the project and my Urdu translator begged to take his name off the book after receiving death threats," she adds.

Her detractors say she is anti-Muslim because she supports the invasion of Iraq and Israel's right to exist. But Manji is adept at answering her critics by throwing Koranic verses and Islamic history back at them. "It was because of itjihad that 135 schools of legal thought flourished in the Islamic medieval world," she argues. "I want to spread itjihad from the west because it is here we are enjoying the freedom to think freely and speak freely."

Any questions? Hands shoot up all over the room. She smiles encouragingly and thanks people for making a "good point", even when she does not agree.

One woman, who may be Iranian, appears rattled and asks, "What do you question? Prayers? Inheritance rights? What?" "All of it," answers Manji. "The Koran is full of ambiguities, like the Bible. I am not trying to undermine the Koran, but to undermine the authoritarianism that seeks to quash debate."

She ends by arguing that there have been several Islamic reformations, but all have been conservative.

Afterwards, everyone ambles out to the reception room to ponder this over glasses of red wine. Manji does not drink.

Several people linger to shake her hand. One middle-aged man says, "In the words of you North Americans, 'You go, girl.'" Manji laughs.