A debate at the Oxford Union was disrupted when protesters forced their way into a building.
Students say the views of the controversial speakers are not valid
BNP leader Nick Griffin and controversial historian David Irving were invited to talk about free speech.
Thirty protesters pushed their way into the debating chamber to stage a protest about the inclusion of the two men.
Between 800 and 1,000 protestors held a demonstration outside the gates of the building but the debate eventually began about one-and-a-half hours late.
However, for safety reasons, the organisers split the debate into two rooms.
Liberal Democrat MP for Oxford West and Abingdon Dr Evan Harris, the journalist Anne Atkins and Mr Irving were in one room.
In the other room were Mr Griffin and two Oxford University post-graduate students.
The two men spoke in separate rooms for safety reasons
Thames Valley Police said the majority of protesters were peaceful, and added that though a "small number" of people entered the Union building, no criminal offences were committed.
Ch Insp Dennis Evernden said: "I am pleased with the way the policing operation went. The vast majority of protesters were peaceful.
"A small minority seemed intent on causing problems but police intervention prevented any criminal acts or disorder."
Anti-racism campaigners had said the two men should not be given a platform to speak at the debate in St Michael's Street, Oxford.
Protesters chanted anti-fascist slogans and jeered "shame on you".
The students broke through a security cordon into the building where the debate, scheduled to start at 2030 GMT, was due to be held.
Union security officers said the protesters got into the building by jumping over the wall while others created a diversion by gathering and crushing at the front gate.
The president of the Oxford Union Debating Society, Luke Tryl, told the BBC he was disappointed by the actions of those who tried to stop the event going ahead.
"The way to take fascism on is through debate and that's how we're going to defeat them," he said.
"David Irving came across looking pathetic. He looked weak. The flaws in his arguments about free speech were exposed and I'm pleased that that happened."
For and against
Martin Mcluskey, from the Oxford University Students' Union, was among those protesting against the inclusion of Mr Irving and Mr Griffin.
He said: "What we are doing here tonight at the Oxford Union is putting them on a platform that will give them legitimacy and credibility.
Police said they were happy with the way the protest was handled
"It is as if we are saying that we agree with what they are saying and that we think it is valid."
But participant Ms Atkins said controversial views should not be silenced but exposed.
"When you say that the majority view is always right I think that is a deeply dangerous and disturbing thing to say.
"I am not for a moment saying that I agree with David Irving or Nick Griffin but I am saying that once you start having truth by democracy you risk silencing some of the most important prophets we have ever had."
Mr Griffin and Mr Irving arrived about four hours before the start of the debate and were escorted into the building.
Student Roland Scarlett, an undergraduate at Oriel College who attended the debate, said most questions were addressed to Mr Irving.
He said: "I think it was a very balanced argument and both sides did really well. I'm pleased it went ahead."
Conservative Dr Julian Lewis, MP for New Forest East, has also resigned his membership of the Oxford debating union in protest describing the two men as "a couple of scoundrels" in his resignation letter.
Mr Griffin has repeatedly insisted the BNP is not a racist group.
He was convicted in 1998 for incitement to racial hatred for material denying the Holocaust.
But in November 2006, Mr Griffin and BNP activist Mark Collett were cleared of a separate accusation of inciting racial hatred after a retrial at Leeds Crown Court.
Mr Irving was jailed in Austria for three years in February 2006 after pleading guilty to Holocaust denial.
He served 13 months of his sentence before being released on probation and was banned from Austria.