A doughty old Jewish friend pushes her way through hundreds of black schoolchildren to greet me in the packed United Palace Theatre. It's an unusual place to find her. Not only is this vast, kitsch playhouse situated in the dodgy streets north of Harlem, where even the doughtiest Jewish dames usually fear to tread. It's now also an evangelical Christian church.

But she's all smiles. “Isn't this something!” she shouts above the din of 400-odd performers warming up. “Lenny would have loved it.”

We are about to hear Leonard Bernstein's Mass, one of the highlights of a three-month festival, Bernstein: The Best of All Possible Worlds, being mounted across New York this autumn by Carnegie Hall and the New York Philharmonic. And what a show it turns out to be. Written for the opening of the Kennedy Centre in Washington in 1971, Mass was vilified by critics for years, and it's not hard to see why. Bernstein turned the Roman Catholic Mass into an existentially anguished Broadway musical - “Kiss Me Kierkegaard,” as the critic Paul Griffiths once put it - with dozens of hoofers straight out of A Chorus Line interrupting the priest with angry rants against God, the Church and religion in general.

In the end the priest himself has a spiritual breakdown and shatters the Cross, which is picked up by an angelic choirboy, who launches the sugary finale. Bernstein once said that his entire output, from West Side Story to his brooding, post-Mahlerian symphonies, was about the dysfunction at the heart of the 20th century: “the crisis of faith”. Mass is its most overt articulation. But like Bernstein himself - a brilliant, charismatic polymath who nevertheless had his naive moments - it's easy to mock.

Not in Harlem last week, though. The performance brought together the Baltimore Symphony Orchestra and a rock band, a gospel choir and 12 of Broadway's liveliest singer-actors - all under Marin Alsop's cool-headed direction. Best of all, however, it involved several hundred New York schoolchildren, who sang, swayed and turned Bernstein's sledgehammer rock riff on “dona nobis pacem” into a cataclysmic massed cry for peace.

My friend is right. Lenny would have loved it. All his life he fought against the ghettoisation of culture into highbrow and lowbrow, youth and middle-aged, black and white, European and American. He himself straddled myriad worlds. This performance, bringing together many disparate communities, seemed the apotheosis of his life and philosophy.

The festival - which includes films, documentaries, Broadway shows and lectures, as well as all his major symphonic and choral works - celebrates three Bernstein anniversaries. He made his sensational debut conducting the New York Philharmonic, deputising at the age of 25 for an ailing Bruno Walter, 65 years ago next week. He became the first American music director of that venerable orchestra 50 years ago. And, had he not succumbed in 1990 to the ravages of an insomniac lifestyle that embraced booze, fags and lashings of bisexual sex with equal fervour, he would have been 90 this year.

New York is right to celebrate him. True, a cynic might argue that, of all 20th-century composers, Bernstein least needs a revival. His music (or at least his musicals) is still part of the cultural fabric on both sides of the Atlantic. West Side Story, On the Town and Candide have all had recent West End stagings.

But Bernstein, more than anyone else, defined New York culturally. Under his direction - though direction is too insipid a word to describe his stamping, leaping, roaring presence - the New York Phil fizzed as it has never fizzed since. He epitomised the city's protean energy, waspish wit, adenoidal croak and liberal politics (the FBI compiled a 660-page dossier on him after his endorsement of the Black Panthers).

And, almost as a hobby, he tossed off at least 20 of the greatest tunes to immortalise the sights and sounds, tribes and tribulations, of Manhattan. He didn't just capture the exuberance and edginess, but also - in songs such as Some Other Time from On the Town, and Somewhere from West Side Story - the loneliness and heartache embedded in a city of immigrants.

The sadness is that, nearly 20 years after his death, nobody has come close to filling the voids he left. No classical composer dares venture into the pop world in the dazzlingly assured way that Bernstein did. No conductor has the magnetism to entice so many newcomers to symphonic masterpieces. And no cultural icon has the breadth of intellect and interests, plus the chutzpah and celebrity, to make such a big splash in the world of politics. Whether taking the Israel Philharmonic into the war zones of the Sinai Desert to play Mozart, or leading a concert for 130,000 people in Central Park, or conducting Beethoven's Ninth at the breaching of the Berlin Wall, Bernstein was more than a musical giant. To New Yorkers, and to millions elsewhere, he was a beacon of humanity in a dark and loveless century.

Bernstein: The Best of All Possible Worlds continues in New York until Dec 13 (www.BernsteinFestival.org)

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