I was just the kind of protester that Sir Tom Stoppard was railing at over the weekend. Posturing rebels, he called us. The generation that took to the streets in Grosvenor Square or the Boulevard St Michel 40 years ago filled him with revulsion.

“I was embarrassed by the slogans and postures of rebellion in a society which, in London as in Paris... seemed to me to be the least worst system into which one might have been born - the open liberal democracy whose very essence was the toleration of dissent,” he wrote.

How odd of Sir Tom, of all people, to have got it so wrong. I can tell him here and now that when the police turned on us in South Audley Street that Sunday - March 17, 1968 - as we marched in protest against the Vietnam War, the last thing on our minds were thoughts of an open liberal democracy.

One moment my friend Joanna and I were strolling in the spring sunshine in Grosvenor Square, admiring Tariq Ali's trademark red coat and Vanessa Redgrave's radical chic headband, the next we were trapped in a narrow causeway with riot police charging down towards us en masse, beating on their riot shields and sending up a roar that, as Joanna said afterwards, made them sound like the Zulu warriors in that Michael Caine film.

Whatever you may say about 1968 - and most of it has been said and said again - toleration of dissent was precisely what was not on offer. In the course of that extraordinary year, a generation that had never particularly questioned authority or thought much about its ideology found itself radicalised by the reaction of those it sought to challenge. Whether on the streets of Berlin, in Washington, on the Left Bank in Paris or in London, what had begun as a disparate series of protests - some dilettante, some serious - were drawn together by a sense that those who exerted power were hopelessly out of touch with those who sought to challenge it.

You cannot, of course, equate a stroll in Grosvenor Square with the Prague Spring or riots on the streets of Warsaw when demonstrators risked and often lost their lives as they fought to throw off the yoke of Soviet oppression, but you can argue that a global movement, mainly of the young, took palpable shape, driven by genuine causes - war in Vietnam or Algeria, Soviet tanks, civil rights in America - drawing strength from its shared experiences, and unsettling the complacency of an Establishment that had never bothered greatly to wonder what the young thought. As the journalist Mark Kurlansky observes in his book 1968 - the Year that Rocked the World, it was “a spontaneous combustion of rebellious spirit”, sparked by a sense of alienation and a shared distaste of authoritarianism.

It was never, as some have suggested, a socialist uprising. Far from it - socialism was dying on its feet. In its most extreme form socialism had by then became the played-out Marxism of Soviet Russia, and if the protesters in Czechoslovakia and Poland were drawn together by anything, it was a feeling that they wanted to share the rewards of Western capitalism rather than reject them. Gene McCarthy, the presidential candidate who came within 300 votes of defeating

Lyndon Johnson in the New Hampshire primary and went on to galvanise that extraordinary election year, was no left-wing firebrand. Bobby Kennedy, another hero of the time, was no champion of radicalism.

And the protest movement, if such it was, had no clear or coherent idea about what shape the politics of change should take. As Mark Rudd, who led the student uprising at Columbia University put it: “I had only the vaguest idea of what we were doing.” Daniel Cohn-Bendit said that his own cheerful brand of street protest had “little discernible ideology”. And Abbie Hoffman, the hippy protester, admitted: “We couldn't agree on lunch.”

But what did unite that generation, and gave it a sense of shared excitement, was the way it was able to challenge authority head on, question its assumptions and force it to reconsider its democratic credentials. In France, de Gaulle's patrician view that he was the very embodiment of the State was shot to pieces by the wave of student protest that won the sympathy if not the support of ordinary people; his reputation never fully recovered. The uprisings in Poland and Czechoslovakia may have come to nought, but they marked the beginning of the end of Soviet power. And while our little demonstration in Grosvenor Square can hardly be said to have stopped the Vietnam War in its tracks, it was nevertheless part of a wave of protest that would in time become an unstoppable tsunami.

It is easy to deride the middle-class rebels who wore Che Guevara T-shirts or worshipped Malcolm X before going on to hold down executive positions or comfortable professorships in the bosom of the Establishment that they had sworn to destroy. But if, by the end of that momentous decade, it was no longer acceptable for a police force to beat dissenters to within an inch of their lives or a communist dictatorship to crush freedom of expression under its boot, then they deserve some of the credit.

More than that, it was a decade when barriers of all kinds - physical, intellectual, social - were torn down, never to be rebuilt. As one French dissident put it: “The real sense of '68 was a tremendous sense of liberation... A whole system of order and authority and tradition was swept aside. Much of the freedom of today began in '68.”

Posturing? Certainly. Embarrassment? Just look at that Viva Zapata moustache. But to call it a protest without a cause, and to suggest it changed nothing is to miss the point of an unforgettable year.

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