Demonstrators gather as Irving refuses to back out of debate

The convicted holocaust denier David Irving is refusing to pull out of tonight's debate on free speech at the Oxford Union, amid fears of widespread unrest in the city.

Hundreds of students and anti-fascist campaigners are expected to protest when Irving and the BNP leader, Nick Griffin, speak at a forum on the limits of free speech.

The decision to invite the pair, made after a vote among members of the debating society, has drawn widespread criticism from student groups and prompted a senior Tory MP, Julian Lewis, to resign his life membership of the union.

There had been some confusion about whether Irving would attend the event but, hours before the meeting was due to start, he told Guardian Unlimited that he would not be put off by political opponents.

"I thought the traditional enemies would have been doing enough to silence me at Oxford. But I have the balls - you'll see what I mean," he said.

At the weekend, Trevor Phillips, who chairs the Equalities and Human Rights Commission, branded the invitation a disgrace.

Anti-fascist groups said they expected hundreds of demonstrators to descend on the city and police have warned the debate's organisers that they fear a number of extreme rightwing activists will stage counter-protests.

The row over the Oxford Union's actions has escalated in the past few weeks, with a number of speakers pulling out of events in protest. The former Europe minister, Denis McShane, the defence secretary, Des Browne, and the television presenter June Sarpong have all cancelled appearances.

In a letter to the union's officers and standing committee, Lewis, MP for New Forest East, said he was resigning his life membership "with great sadness".

In his resignation letter, he said: "Nothing which happens in tonight's debate can possibly offset the boost you are giving to a couple of scoundrels who can put up with anything except being ignored."

But the Liberal Democrat MP Evan Harris, who is speaking at the event, said banning Griffin and Irving would risk turning "bigots into martyrs".

Luke Tryl, the president of the union, said: "I find the views of the BNP and David Irving awful and abhorrent but my members agreed that the best way to beat extremism is through debate.

"It would have been much easier for me to have a term as president in which I didn't try to uphold this principle."

A spokesman for the BNP said Griffin was determined to attend and would be accompanied by his personal security team, but added that the far right organisation was not planning a big demonstration outside the venue.

A spokeswoman for Thames Valley police said that the event and demonstration would be policed "like any other protest".

Chief inspector Dennis Evernden said: "Our role in this event is to prevent any breach of the peace, facilitate any lawful protest and reassure the general public. We are very used to dealing with protest in Oxford and will deal with this event in a sensitive and professional manner."