Boris Johnson's mayoral campaign was dealt a blow last night as he was booed and heckled at the biggest hustings of the race so far.

The Tory candidate was faced with a hostile audience of 2,500 Londoners including church groups, trade unions and charities.

But by the end of the two-hour event at the Methodist Central Hall in Westminster the jeers had turned to cheers as he won round much of the audience.

Mr Johnson was visibly nervous when he took to the stage at the London Citizens hustings, jiggling his foot and bearing a grimace-like smile. He was met with whistles and boos as he stood up to speak, admitting: "This is the most wonderful and intimidating event I've ever been at."

The audience was packed with black church groups and student and trade unions who are the natural constituency of Labour rival Ken Livingstone.

Mr Johnson went on to talk about the high cost of living in the city, including the mayoral precept of the council tax, but was barely audible amid the noise.

Eventually one of the event organisers interrupted and told the gathering: "I'm sorry but I've got to interrupt here. In the interests of respect and dignity, we'll be giving candidates extra time if they're heckled and booed."

The audience began to warm to Mr Johnson after he agreed to fund the "London living wage" of £7.20 per hour for the poorest workers if elected.

He won over even more people when he talked about housing and agreed to a one-off amnesty for all illegal immigrants living in the capital.

Mr Johnson spoke of his own family's immigrant roots and said his Muslim great-grandfather, who fled to Britain from Turkey, would be "very proud" he was standing for Mayor of London.

The candidate said: "If an immigrant has been here for a long time and there is no realistic prospect of returning them, then I do think that person's condition should be regularised so that they can pay taxes and join the rest of society."

Mr Livingstone added it was a "tragic miscalculation" by the Labour government not to have an "immediate amnesty for everybody" when it came to power in 1997. However, the Tory faced jeers when he said it was not within his powers to stop the Met staging controversial dawn raids of migrant families. "I've given you as many yeses as I can, my friends," he implored his audience. He added that he would "look at" London Citizens' proposal to subsidise transport for failed asylum seekers in London, while Green Sian Berry and Liberal Democrat Brian Paddick backed the idea. Mr Johnson was then applauded when he repeated his pledge to scrap the Mayor's newspaper, The Londoner, and plant trees with the money saved.

Later, Tory aides attempted to put a positive spin on the evening, claiming that despite the rocky start, by the end he had convinced many attendees. One insider said: "Despite a trade union block booing Boris at the start he seemed to have turned some heads tonight. It took us more than half an hour to get out of the venue as he was mobbed for autographs and photos at the end."

It comes after the Tories were accused of keeping Mr Johnson out of the public eye by avoiding events with potentially difficult audiences including ethnic minorities and Labour supporters. He has vehemently denied he has been avoiding tricky hustings.

Mr Livingstone also attracted controversy at the hustings when he agreed to give failed asylum-seekers free travel so they could attend immigration interviews. Many end up walking for miles across the capital because they are unable to afford the Tube or train fare. "There may be a problem with the travel, but I'll think of a way round it," the Mayor said.

He suggested the scheme could be funded by dipping into the money saved as a result of a cheap fuel deal with president Hugo Chavéz of Venezuela.

The Labour Mayor said: "[Mr Chavéz] has given us £14 million that allows people on benefits to get half-price travel on the bus. I would ask him if he is prepared to amend the scheme to cover failed asylum seekers."

Mr Paddick and Ms Berry both gave confident performances. The Lib Dem sang along with the gospel choir and tapped his foot when he took to the stage.

He told the audience: "What I'm offering you is change and somebody who is not interested in talking but is actually interested in getting things done."

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