Police were last night criticised for allowing protesters to invade the Oxford Union, attack union members and hurl death threats.

About 50 protesters gatecrashed the Oxford Union's debate with holocaust denier David Irving and BNP leader Nick Griffin in St Michael's Street on Monday, with 20 staging a sit-in.

Union president Luke Tryl and Oxford MP Evan Harris said officers should have stopped the protesters storming the gate to the Union building and disrupting the debate.

But Supt Brendan O'Dowda, the head of Oxford police, said his officers fulfilled their responsibility in policing a peaceful protest outside.

Up to 1,000 people - most protesting against the presence of Mr Griffin and Mr Irving - took part in the demonstration. Mr O'Dowda said the police operation was a success because there was no outbreak of mass disorder and no-one was arrested.

Mr Tryl said: "Members of the (Union) committee were punched and although I'm not sure arrests should have been made, the shouts of 'Kill Tryl' were incitement of violence. It just shows the laissez faire attitude of the police.

"I feel they didn't want the event to go ahead and didn't give us an adequate level of support.

"When you have people climbing on walls and trying to break down the gate, that is a police responsibility."

Dr Evan Harris, the Oxford West and Abingdon MP who spoke at the debate, said: "I am disappointed police were not able to separate hundreds of young people having a lawful private meeting from some of the hundreds of protesters who were intent on preventing a lawful meeting taking place."

Mr Griffin added: "I think it's wrong that the police didn't deal with it more effectively."

One first-year student from Wadham College said: "The people protesting out here and climbing over the walls were absolutely ridiculous.

"There should be some kind of police complaints investigation."

Mr O'Dowda said a memorandum before the event set out agreements for his officers only to police the protests outside the Oxford Union premises.

He added: "People may be quick to point the finger at the police, but it wasn't the police that organised this event. The Oxford Union organised this event, and like any demonstration the length and breadth of this country, the police have to be put in a position to facilitate a lawful protest.

"It was not our problem, or an issue for us, if a small group of people got inside a private event in a private chamber - that's a matter for those organising the event."

Officers did enter the building, but only to assist in escorting the protesters from the chamber by the Union's security.

Mr O'Dowda said they did not commit any criminal offence and any allegations of trespass must be taken up by the union.

He added: "Our responsibility was not to facilitate through traffic in and out of the Oxford Union."