In June 1996, armed police surrounded a Kurdish community centre in Haringey, north London. As a helicopter hovered overhead, marksmen on the rooftops trained their rifles on the entrances and exits, writes Fintan O'Toole

Anyone emerging was seized, handcuffed, and forbidden to communicate in Kurdish or Turkish. After an hour, the police smashed down the doors and stormed inside. There they found props and scripts for Harold Pinter's play about culture, torture and subjugation, Mountain Language. The armed and masked men, who worried residents had reported entering the building, were actors. Their guns were plastic imitations. As Pinter told the Guardian at the time, "the line between fiction and reality sometimes becomes very blurred".

It has certainly become increasingly blurred in Pinter's own plays. For a long time, the dark, strange, apparently enclosed fictions of his theatre seemed utterly distant from public political realities. Their characters, as the audience experiences them, are inventing not just stories, but selves. They have no offstage lives. They have no interest in convincing us of their own reality, let alone of any particular proposition about the real world. Yet, as time has gone on, and Pinter's work has settled into the collective consciousness, it has become ever more obvious that it is, in the broadest sense, realistic. It adds the cruelties of political repression to the malice and unkindness of human relationships and gives us, in sum, an all-too-recognisable world.

That audiences would come to recognise the public and private realties of Pinter's plays would have puzzled those who helped to create the early 20th century's brand of political theatre. For the older generation of committed left-wing playwrights such as Sean O'Casey - who in 1964 attacked him almost with his dying breath in the last article he ever wrote - Pinter's strange dialogue ("like the hammering of a woodpecker's beak against the trunk of a tree") and apparent refusal of public meanings seemed like mere decadence.

In part, such critics were taken in by Pinter's own efforts to deflect too literal a reading of his plays by disavowing any real interest in the public world. In 1966, for example, Pinter told the Paris Review, "I don't think I've got any kind of social function that's of any value, and politically there's no question of my getting involved because the issues are by no means simple - to be a politician you have to be able to present a simple picture even if you don't see things that way . . . I don't feel threatened by any political body or activity at all . . . I don't care about political structures . . . "

What has happened in the meantime is partly that Pinter has become much more explicitly political, but partly, too, that the central concerns of politics have become much more explicitly Pinteresque. Radical politics have focused much more clearly on cruelty, violence, arbitrary power, and torture. As the abstract and schematic Utopianism of the 1960s has wilted, the brute facts of what people do to other people's minds and bodies have taken their place at the centre of political concern. And those facts are, and always have been, Pinter's territory. It has become clear, in other words, that what used to be called Theatre of the Absurd is sheer realism. It has less to do with existentialism or the death of God than with the pervasiveness of political terror.

The origin of Pinter's concern with terror is far from mysterious. When I interviewed him for the Irish Times in 1994, he recalled being sharply aware of the Holocaust as an adolescent and seeing photographs and newsreels of the concentration camps: "By the time the war ended I was 15 and a half. I wasn't a child any more. And then immediately the war ended, on top of that, the fascists came out again in London, which again is not a fact that is generally acknowledged. So I ran into them straight away between the ages of 16 and 17. Once again the taunt 'Jewboy' came up and related itself to what had just happened in Europe."

The Holocaust, however, was hard to represent without bathos, and some intellectuals argued that the only proper response to it was silence. Pinter's achievement has been to make that dumbfounded silence and the awful absence of annihilation present to us on stage. He has taken what is too large to be stated directly and filtered it through the apparently small and banal.

Watching a Harold Pinter play for the first time is rather like the moment in maths class when the teacher introduces the idea that numbers can be minus as well as plus. It seems at first a ridiculous notion - how can anything be less than zero? - but because this absurdity follows all the forms and rules that you have been taught to expect, you learn to accept it. Pinter's plays work the same way - starting with next to nothing and working down. But because they do so with perfect logical precision, you find, as a member of the audience, that they have shape and coherence. Pinter's genius has been to construct his plays as if they were of the usual sort, as if all the familiar rules still applied. The outward form of naturalistic theatre is maintained, even though its inner core - cause and effect - has been removed.

In this, the plays are strongly reminiscent of MC Bradbrook's description of the action of one of the great Jacobean tragedies, Cyril Tourneur's The Revenger's Tragedy: "always just logically possible, like a detective story, though impossible by any other standards". They have the shape and feel of well-made plays in which every effect has a cause and every action produces a reaction. But the things they describe - violence, terror, disintegration - are usually dark and irrational.

Stripped of so many of the things that normally make theatre interesting, all the force of Pinter's plays lies in what is not said, in what remains, beyond the confines of the room where the action takes place, unseen and unheard. Most plays begin, from the point of view of the audience, with a total ignorance that is gradually reduced. Pinter's begin with a total ignorance that is gradually increased. Instead of moving toward knowledge - who these people are, what they think and feel, why they do what they do, how they end up - we move deeper into mystery.

The people on stage decline to do what characters in the theatre usually do - fill in the gaps in our knowledge, impart information, construct a story. We know nothing about them except what they say from moment to moment. We come to understand that the words they use are designed not to communicate with us or with each other but to avoid communication. They do not ask for our sympathy or our understanding and we have little opportunity for empathy. They do not offer enlightenment or uplift. But as they speak and move, they summon up what is not there: the great unspeakable darkness of man's inhumanity to man.