There is an anarchist leftist group here in London who hate me," Slavoj Zizek says with a giggle. "But let us speak frankly, most of the left hates me even though I am supposed to be one of the world's leading communist intellectuals."

He then launches into a denunciation of the pretensions of James Joyce and then into a eulogy to the radical minimalism of Beckett's Not I. Within minutes we are on to German philosopher Peter Sloterdijk's views on the Malaysian economic miracle, the prospects for Zizek's film theory course in Ramallah and Katarina Wagner's production of Die Meistersinger von Nurnberg.

Back to those shadowy anarchists. It was they (his PR people suggest a student group armed with a fake Facebook account rather than fizzing bombs) who smeared him online, claiming he was having a "thing" with singer Stefani Joanne Angelina Germanotta, aka Lady Gaga. It was, of course, a hoax, but sections of what Sarah Palin calls the lamestream media ran with it, including the New York Post and the London-based Daily Star, the latter reporting: "Pals fear the Lady Gaga's head is being filled with extremist ideas by Slovenia-born Slavoj Zizek."

What irked the not-Lord Gaga was not so much the unwarranted Hegelian dialectical inversion (surely he might more plausibly have been corrupted by her extremist ideas?) but that the fake Facebook page claimed that Lady Gaga and Zizek cemented their relationship by deconstructing patriarchal ideology, feminism and collective human responsibility. It was an intolerable slur: "I don't say those sort of things. Can you imagine a more boring evening?"

How would you have spent an evening with Lady Gaga? He chuckles but waves away the question (Zizek does many things in conversation but answering questions is not one of them).

"My mistake was that I should not have categorically denied a relationship to the press. I should have said ‘no comment', leaving a gap for the obscene possibility that I am her lover."

The 62-year-old Slovenian Lacanian Hegelian is in London not to confront importunate anarchists but to promote the paperback edition of his book Living in the End Times and to share a platform with Julian Assange to discuss the meaning of WikiLeaks.

The book, which he denounces heartily, represents the best and worst of his thought: Its flashes of genius highlight a mind that seems incapable of following a thought for more than a page and a half.

His performance with Assange and radical American journalist Amy Goodman at the Troxy Theatre in east London proved better — part pomposity-deflating vaudeville turn and part devastating critique of contemporary capitalism. "I have to subvert these events," he tells me. "The pious questions, the solemn speeches. My God, how can you sit through these things without wanting to make a joke?"

It is worth watching the scene on YouTube just to register how Assange contains his disgust. Assange, to his credit, also kept his composure when Zizek called him a terrorist. "You are a terrorist in the way that Gandhi was. In what sense was Gandhi a terrorist? He tried to stop the normal functioning of the British state in India. You are trying to stop the normal functioning of information circulation."

Zizek was serious. He wrote about it in an essay, arguing against a liberal interpretation of WikiLeaks that reduces its impact to "a radical case of ‘investigative journalism'. Here, we are only a small step away from the ideology of such Hollywood blockbusters as All the President's Men and The Pelican Brief, in which a couple of ordinary guys discover a scandal which reaches up to the president, forcing him to step down. The ideology of such works resides in their upbeat final message: what a great country ours must be, when a couple of ordinary guys like you and me can bring down the president, the mightiest man on Earth!"

"We learnt nothing new, really, from WikiLeaks," he tells me later. "Julian is like the boy who tells us the emperor is naked — until the boy says it everybody could pretend the emperor wasn't. Don't confuse this with the usual bourgeois heroism which says there is rottenness but the system is basically sound."

Zizek sips hot chocolate and wipes his beard. "I should not, speaking frankly, be this man who talks about The Dark Knight and Hegel, about the value of WikiLeaks and Lady Gaga. I should be a mediocre philosophy professor in Ljubljana."

The blurb for his new books says he has made philosophy relevant for a whole generation of politically committed readers. Zizek demurs. "A lot of what I write is blah, blah, a diversion from the 700-page book on Hegel I should be writing."

In 2009, responding to a call for a reconsideration of communism by his friend, Parisian philosopher Alain Badiou, Zizek took part in a London conference to test the notion that capitalism was on the point (yet again) of falling apart from its own contradictions and so theorising the emancipated future was imperative. He went on to co-edit The Idea of Communism, a book urging lapsed comrades to raise the red flag anew. "Don't be afraid, come back!" Zizek wrote. "You've had your anti-communist fun, you are pardoned for it — time to get serious again!"

It is time for him to leave. "My son and I are going to see Transformers." He means the third and final instalment of the dismal film franchise. It is terrible, I warn him. "I have been to terrible films before. There is always something worth seeing."