A crowd of some 300 jeering, banner-waving demonstrators delayed by more than an hour a controversial speech by Marine Le Pen, the leader of France’s far-right National Front, to the Oxford Union on Thursday.

Security guards were forced to close the doors to the university’s debating society after a dozen balaclava-wearing anti-fascist protesters, chanting “Le Pen, never again”, “No pasarán” and “Oxford Union, shame on you,” came close to scaling the walls from the street outside.

Smuggled in through a side door two hours before the scheduled start of her speech, Le Pen said she was very grateful for the invitation to address an organisation that “since its foundation has been a place of open debate and freedom of expression”.

In the aftermath of last month’s terrorist attacks in Paris in which three Islamist gunmen shot dead 17 people, including 12 at the offices of the satirical magazine Charlie Hebdo, she said it was clear that the “first of all our freedoms, security, is now threatened by Islamist terrorism”.

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She said it was essential that France regain control of its borders so as to halt the traffic of illegal arms used in the Charlie Hebdo massacre and to “bring a halt to mass immigration.” With 6.1m unemployed, France “can simply no longer welcome” new immigrants, she said.

Demanding that Muslims in France abide by the country’s secular rules, Le Pen added that the Front National had “never accepted the idea of a separate community within the Republic”. Combating Islamism meant France had “to reaffirm our principles: secularism, assimilation, the rule of law”.

Le Pen took over from her father, Jean-Marie, as leader of the Front National in 2011 and has since made strenuous efforts to pilot the nationalist, anti-immigrant and once openly racist party into the mainstream of French politics.

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Last year’s European elections saw it finish top of a national poll for the first time in its 40-year history and polls predict Le Pen will comfortably make it through to the second round run-off in presidential elections due in 2017.

Demonstrators outside were bitter that she had been asked to address the Union. “This isn’t about freedom of speech – she has the right to express her views; no one is trying to silence her,” said Barnaby Raine, a second-year history and politics student and one of the protest organisers.

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“But that doesn’t mean we have to invite her here to give an hour-long talk and bring her bigoted, divisive politics into our community. Fascism has only ever been beaten when good people mobilise against it. Not by inviting it to dinner.”

John Tanner, an Oxford councillor, said the society’s invitation was a “stunt”. “The people of Oxford don’t want this extreme right-wing racist from France given a platform here,” he added.

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But students who queued in freezing temperatures to hear Le Pen speak disagreed. “This protest is ironic and counter-productive,” declared Benjamin Evans, studying politics, philosophy and economics.

“Ironic because stifling debate is the essence of fascism, and counterproductive because the way to challenge these ideas is by confronting them in open and public debate.”

Maddy Wild, a geography student, said: “I completely disagree with basically everything Marine Le Pen says. But freedom of speech and expression is essential in a democracy.”

But Annie Teriba, a history and politics student, said she was disgusted that Le Pen could “now go back to France and say she has been invited to speak at Oxford University. That is the kind of legitimacy that is allowing her and her abhorrent party to become acceptable.”