Noam Chomsky is the closest thing in the English-speaking world to an intellectual superstar. A philosopher of language and political campaigner of towering academic reputation, who as good as invented modern linguistics, he is entertained by presidents, addresses the UN general assembly and commands a mass international audience. When he spoke in London last week, thousands of young people battled for tickets to attend his lectures, followed live on the internet across the globe, as the 80-year-old American linguist fielded questions from as far away as besieged Gaza.

But the bulk of the mainstream western media doesn't seem to have noticed. His books sell in their hundreds of thousands, he is mobbed by students as a celebrity, but he is rarely reported or interviewed in the US outside radical journals and websites. The explanation, of course, isn't hard to find. Chomsky is America's most prominent critic of the US imperial role in the world, which he has used his erudition and standing to expose and excoriate since Vietnam.

Like the English philosopher Bertrand Russell, who spoke out against western-backed wars until his death at the age of 97, Chomsky has lent his academic prestige to a relentless campaign against his own country's barbarities abroad – though in contrast to the aristocratic Russell, Chomsky is the child of working class Jewish refugees from Tsarist pogroms. Not surprisingly, he has been repaid with either denunciation or, far more typically, silence. Whereas a much slighter figure such as the Atlanticist French philosopher Bernard Henri-Lévy is lionised at home and abroad, Chomsky and his genuine popularity are ignored.

Indeed, his books have been banned from the US prison library in Guantánamo. You'd hardly need a clearer example of his model of how dissenting views are filtered out of the western media, set out in his 1990's book Manufacturing Consent, than his own case. But as Chomsky is the first to point out, the marginalisation of opponents of western state policy is as nothing compared to the brutalities suffered by those who challenge states backed by the US and its allies in the Middle East.

We meet in a break between a schedule of lectures and talks that would be punishing for a man half his age. At the podium, Chomsky's style is dry and low-key, as he ranges without pausing for breath from one region and historical conflict to another, always buttressed with a barrage of sources and quotations, often from US government archives and leaders themselves.

But in discussion he is warm and engaged, only hampered by slight deafness. He has only recently started travelling again, he explains, after a three-year hiatus while he was caring for his wife and fellow linguist, Carol, who died from cancer last December. Despite their privilege, his concentrated exposure to the continuing injustices and exorbitant expense of the US health system has clearly left him angry. Public emergency rooms are "uncivilised, there is no health care", he says, and the same kind of corporate interests that drive US foreign policy are also setting the limits of domestic social reform.

All three schemes now being considered for Barack Obama's health care reform are "to the right of the public, which is two to one in favour of a public option. But the New York Times says that has no political support, by which they mean from the insurance and pharmaceutical companies." Now the American Petroleum Institute is determined to "follow the success of the insurance industry in killing off health reform," Chomsky says, and do the same to hopes of genuine international action at next month's Copenhagen climate change summit. Only the forms of power have changed since the foundation of the republic, he says, when James Madison insisted that the new state should "protect the minority of the opulent against the majority".

Chomsky supported Obama's election campaign in swing states, but regards his presidency as representing little more than a "shift back towards the centre" and a striking foreign policy continuity with George Bush's second administration. "The first Bush administration was way off the spectrum, America's prestige sank to a historic low and the people who run the country didn't like that." But he is surprised so many people abroad, especially in the third world, are disappointed at how little Obama has changed. "His campaign rhetoric, hope and change, was entirely vacuous. There was no principled criticism of the Iraq war: he called it a strategic blunder. And Condoleezza Rice was black – does that mean she was sympathetic to third world problems?"

The veteran activist has described the US invasion of Afghanistan as "one of the most immoral acts in modern history", which united the jihadist movement around al-Qaida, sharply increased the level of terrorism and was "perfectly irrational – unless the security of the population is not the main priority". Which, of course, Chomsky believes, it is not. "States are not moral agents," he says, and believes that now that Obama is escalating the war, it has become even clearer that the occupation is about the credibility of Nato and US global power.

This is a recurrent theme in Chomsky's thinking about the American empire. He argues that since government officials first formulated plans for a "grand area" strategy for US global domination in the early 1940s, successive administrations have been guided by a "godfather principle, straight out of the mafia: that defiance cannot be tolerated. It's a major feature of state policy." "Successful defiance" has to be punished, even where it damages business interests, as in the economic blockade of Cuba – in case "the contagion spreads".

The gap between the interests of those who control American foreign policy and the public is also borne out, in Chomsky's view, by the US's unwavering support for Israel and "rejectionism" of the two-state solution effectively on offer for 30 years. That's not because of the overweening power of the Israel lobby in the US, but because Israel is a strategic and commercial asset which underpins rather than undermines US domination of the Middle East. "Even in the 1950s, President Eisenhower was concerned about what he called a campaign of hatred of the US in the Arab world, because of the perception on the Arab street that it supported harsh and oppressive regimes to take their oil."

Half a century later, corporations like Lockheed Martin and Exxon Mobil are doing fine, he says: America's one-sided role in the Middle East isn't harming their interests, whatever risks it might bring for anyone else.

Chomsky is sometimes criticised on the left for encouraging pessimism or inaction by emphasising the overwhelming weight of US power – or for failing to connect his own activism with labour or social movements on the ground. He is certainly his own man, holds some idiosyncratic views (I was startled, for instance, to hear him say that Vietnam was a strategic victory for the US in southeast Asia, despite its humiliating 1975 withdrawal) and has drawn flak for defending freedom of speech for Holocaust deniers. He describes himself as an anarchist or libertarian socialist, but often sounds more like a radical liberal – which is perhaps why he enrages more middle-of-the-road American liberals who don't appreciate their views being taken to the logical conclusion.

But for an octogenarian who has been active on the left since the 1930s, Chomsky sounds strikingly upbeat. He's a keen supporter of the wave of progressive change that has swept South America in the past decade ("one of the liberal criticisms of Bush is that he didn't pay enough attention to Latin America – it was the best thing that ever happened to Latin America"). He also believes there are now constraints on imperial power which didn't exist in the past: "They couldn't get away with the kind of chemical warfare and blanket B52 bombing that Kennedy did," in the 1960s. He even has some qualified hopes for the internet as a way around the monopoly of the corporate-dominated media.

But what of the charge so often made that he's an "anti-American" figure who can only see the crimes of his own government while ignoring the crimes of others around the world? "Anti-Americanism is a pure totalitarian concept," he retorts. "The very notion is idiotic. Of course you don't deny other crimes, but your primary moral responsibility is for your own actions, which you can do something about. It's the same charge which was made in the Bible by King Ahab, the epitome of evil, when he demanded of the prophet Elijah: why are you a hater of Israel? He was identifying himself with society and criticism of the state with criticism of society."

It's a telling analogy. Chomsky is a studiedly modest man who would balk at any such comparison. But in the Biblical tradition of the conflict between prophets and kings, there's not the slightest doubt which side he represents.