"There is an anarchist leftist group here in London who hate me," says Slavoj Žižek with a giggle as we settle into a dilapidated leather sofa in the bar of his Bloomsbury hotel. He is wearing freebie airline socks, an Italian T-shirt someone gave him and jeans that could easily have been made decades earlier in an unsuccessful Soviet tractor factory. "But fuck it, let's speak frankly, no bullshit, most of the left hates me even though I am supposed to be one of the world's leading communist intellectuals."

Žižek summons the waiter and orders hot chocolate, Diet Coke and lots of sugar ("I am diabetic"). He is disappointed, he tells me parenthetically, that we didn't do the interview in the hotel's adjacent Virginia Woolf burger bar. "What would the Virginia Woolf burger be like?" he asks. "Dried out, topped with parsley, totally overrated. I always preferred Daphne du Maurier." He then launches into a denunciation of the pretensions of James Joyce, arguing that his literary career went downhill after Dubliners, and then into a eulogy to the radical minimalism of Beckett's Not I. Within minutes we're on to German philosopher Peter Sloterdijk's views on the Malaysian economic miracle, the prospects for Žižek's film theory course in Ramallah and Katarina Wagner's production of Die Meistersinger von Nürnberg, in which Hans Sachs is depicted as a Heil Hitler-ing Nazi. One's task as a reader or interviewer of Žižek is rapidly to build a network of mental pontoon bridges to unite his seemingly autonomous intellectual territories.

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 our own Daily Star, the latter reporting: "Pals fear the Lady Gaga's head is being filled with extremist ideas by Slovenia-born Slavoj Žižek."

What irked the not-Lord Gaga wasn't 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 Žižek 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 (Žižek does many things in conversation but answering questions isn't 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." There may be a gap in his love life: he was formerly married to Slovenian philosopher Renata Salecl and to Argentine model and Lacanian scholar Analia Hounie, but declines to tell me if they have a current successor. He has two sons, one in his early 30s, the other 10 years old.

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. He is professor at the European Graduate School in Switzerland, international director of the Birkbeck Institute for the Humanities, University of London; senior researcher at the Institute of Sociology, University of Ljubljana and regularly a visiting professor to some American university or other. Žižek is rarely at home in his flat in the Slovenian capital: he is philosophy's answer to Bob Dylan, frontman of a live roadshow that shows no sign of ending.

The book, which he denounces heartily ("I wrote about Avatar before I'd seen the film, but having seen it I was right to attack it . . . The suggestion that capitalism is ready to collapse is perhaps, I admit it, wishful thinking"), 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 afterwards. "The pious questions, the solemn speeches. My God, how can you sit through these things without wanting to make a joke?" About 40 minutes into the event he yielded to temptation and mutated briefly into Frankie Boyle: there is some good news and some bad news, a doctor tells a husband. Your wife is alive. The bad news is that she has anal and vaginal leakage so bad that she cannot have sex. The husband is revolted. The punchline? The doctor is only joking – the good news is that his wife is dead. It's worth watching the scene on YouTube just to register how Goodman and Assange heroically contained their disgust. Assange, to his credit, also kept his composure when Žižek 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."

Žižek was entirely serious. He wrote about it in a fine 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. Corruption is shown to reach the very top, yet 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 learned 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. It is like a man who finds his wife has been fucking around – until he can see in great detail what she has been doing, he can pretend to himself nothing is wrong. Julian strips away that pretence. All power is hypocritical like this. What power finds intolerable is when the hypocrisy is revealed."

Žižek 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." He was born on 21 March 1949 in the Slovenian capital in what was then Yugoslavia to a father (Jože) who was an economist and mother (Vesna), an accountant. He had an unhappy childhood. "I read alone, a Freudian retreat that prepared me for the world in all its disgusting obscenity." He glances at me with a jaunty expression: "I trust that when you write this you will not be the usual shitty journalist who is true to the facts. I am expecting creative distortion of my biography. Truth is overrated: I have always been happiest alone. Why should that be interesting to read about?"

He underestimates his life. As a teenager Slavoj wanted to be a film director, but set that ambition aside after being seduced by Hegel. Like other budding Slovenian philosophers, he was influenced by Marxist philosopher Božidar Debenjak's Frankfurt School-inflected lectures of Marx's Das Kapital from the perspective of Hegel's Phenomenology of the Mind. "Hegel is everything to me. His collected works remain my most treasured possession," he says. But he was seduced later by French post-structuralists – Derrida, Foucault, Kristeva and, above all, Jacques Lacan, the psychoanalytic theorist. He was fired as an assistant researcher at the University of Ljubljana when his PhD was initially rejected for being non-Marxist. He spent four years doing national service and then another four unemployed before getting a job as a recording clerk at the Slovenian Marxist Centre, where he became involved with scholars committed to Lacanian psychoanalysis.

He spent the early 1980s in Paris, studying psychoanalysis with Jacques-Alain Miller and François Regnault, before returning to Slovenia where he joined dissident groups critical of Tito's regime. "I was a member of the Communist party until 1988 when it became disgusting to remain in a party that defended militarism." After Tito fell, Žižek – by then a celebrated figure in his homeland as columnist for alternative youth magazine Mladina, and as a leading member of the Committee for the Defence of Human Rights – decided to run as a candidate for the presidency of the Republic of Slovenia in the first free elections, in 1990. He stood for the Liberal Democratic party and came fifth. "Politics," he reflects, "has always been shitty. It is something I am involved in often against my will. My first interest is theory. I am a Hegelian looking for facts to fit the theory."

During this period he developed his literary style in dissident magazines and journals, with Marxist, Hegelian and Lacanian thought juxtaposed with critical analyses of cinema and popular culture in a sometimes appealing sometimes exasperating written equivalent of jazz improvisation. His first book to appear in English, 1989's The Sublime Object of Ideology, brought that style to a broader audience. It used examples from high and low culture in order to explain his understanding of Hegel's dialectic, the basic thesis that underpins all his analyses, and one which finds that contradiction is an internal condition of every identity. Contradiction was also the internal condition of Slavoj Žižek, bearing out Oscar Wilde's dictum: "The well bred contradict other people. The wise contradict themselves." "My thinking moves so quickly how could it not be full of contradictions?" he asks.

More books in English followed in rapid succession, including For They Know Not What They Do (1991, a book framing the re-emergence of militant nationalism and racism in the former socialist countries of Eastern Europe as a Lacanian eruption of enjoyment), Tarrying with the Negative: Kant, Hegel and the Critique of Ideology (1993), Welcome to the Desert of the Real (2002), The Parallax View (2006) and In Defence of Lost Causes (2008).

These books (and several others) earned Žižek much praise. Terry Eagleton called him "the most formidably brilliant exponent of psychoanalysis, indeed of cultural theory in general to have emerged from Europe in some decades". Film-maker Sophie Fiennes, who directed him in The Pervert's Guide to the Cinema, a 2005 Channel 4 TV documentary in which he presents marvellously Lacanian analyses of some of his favourite films, says: "He is very much a thinker for our turbulent, high speed, information-led lives precisely because he insists on the freedom to stop and think hard about who you are as an individual in this fragmented society." The Chronicle of Higher Education dubbed him "the Elvis of cultural theory".

The blurb for his new books says he has made philosophy relevant for a whole generation of politically committed readers. Žižek demurs. "A lot of what I write is blah, blah, bullshit, 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, Žižek 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, join us, come back!" Žižek wrote. "You've had your anti-communist fun, and you are pardoned for it – time to get serious once again!"

Are you or have you ever been a communist? "Not as you might imagine. Marx wrote about the commons – he meant land and property. I mean information. When we pay rent to Bill Gates that is a new kind of enclosure. WikiLeaks represents a threat to such control of information."

Do you really believe in such a society? "I am a philosopher not a prophet. I don't answer questions but ask them to critique our society. Benjamin said it is the task of the leftist thinker not to ride the train of history but to apply the brake. It is also important not to say what everybody else is saying. It is boring, for instance, to criticise the US eternally. Why not China instead? It is China, after all, where they have banned fictional works considering alternative worlds, because they are afraid of their citizens' imaginations. It is China that is colonising Africa."

In Living in the End Times, he imagines what a new communist society would be like. He considers both the 2006 NBC TV series Heroes and Theodor Sturgeon's 1953 novel More Than Human as prototypes. Both have the "motif of the alternative community of freaks, where a group of outcasts form a new collective" based on their uncommon psycho-physical abilities (telekinesis, telepathy and so on). Žižek writes: "Their coming together as a new One creates the conditions for their peculiarities to flourish. Does this weird collective not recall Marx's claim that, in a communist society, the freedom of all will be grounded in the freedom of every individual?" Like Marx's account of communist society, it's surely too sketchy to found a political platform. "I am utterly pessimistic about the future, about the possibility of an emancipated communist society. But that doesn't mean I don't want to imagine it."

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's apparently terrible, I warn him. "I have been to terrible films before. There is always something worth seeing."