Oxford University was preparing for lockdown tonight ahead of threatened protests against the decision to invite two controversial far right figures to a debate.

Nick Griffin, the British National Party leader, and the historian David Irving, who was jailed in Austria for Holocaust denial, are due to speak at the Oxford Union debating society on the subject of free speech.

Oxford colleges e-mailed their students warning them to stay in their rooms, and many colleges were planning to lock their doors this evening, amid fears that there could be a counter-demonstration by far-right activists which could turn violent.

Protesters planning to demonstrate against the debate include several university societies such as the Student Union, Unite Against Fascism, as well as a rare alliance between the Muslim and Jewish societies. Scores of students were also being bussed in from other universities.

More than a thousand are expected to attend, and colleges today sent their students e-mails warning them to stay indoors during the protest. Students fear that a counter-demonstration by far-right activists could set off scuffles.

Mr Griffin, who received a suspended prison sentence in 1998 for incitement to racial hatred for material denying the Holocaust, and Mr Irving, who spent three years in prison in Austria for Holocaust-denial, are due to take part in a discussion on the limits of free speech.