Trevor Nunn has triumphantly rescued George Gershwin's famous black folk opera from its familiar, upmarket billets at Covent Garden and Glyndebourne, reconceiving it as a musical for the people.

The first-night audience, not all of whom can have had financial or personal investment in Sir Trevor's mighty project, duly rewarded The Gershwins' Porgy And Bess, as it is entitled, with an instant standing ovation. It is not hard to see why they were so excited.

Sir Trevor's ardent production, in which Nicola Hughes's sultry prostitute Bess is torn or at least pulled between her sexy pimp Crown, her supplier of angeldust and Clarke Peters's crippled Porgy, works a rare treat for anyone who savours high and low culture smoothly blended.

The show's intoxicating mix of sex and love in a black South Carolina fishing community, beset by drug-taking, dicing and double murder, slips smoothly into modern operatic or musical form. I Got Plenty O' Nuttin' or My Man's Gone Now can sound beautiful either as arias or numbers.

Nunn's adaptation of the piece as music theatre slices more than an hour off the opera's four-hour running time. This is no dumbing-down exercise, more a revitalising effort. The original operatic recitative has been banished, replaced with more natural dialogue borrowed from the book and play on which Porgy And Bess was based.

Gareth Valentine has adapted Gershwin's score, transposing it to a key suitable for actors in a musical rather than opera singers. He has slashed the opera's 55-strong orchestra down to a musicals band of 25 with synthesisers. The jazzy, gospel sound is beautifully modulated and a few lost songs restored. You can actually hear the singers and their words, especially the surprisingly powerful Peters and Hughes.

John Gunter's design, based mainly on a wooden tenement building on Catfish Row and moving out to bar-room, ballroom and Kittiwah Island, looks suitably raffish and run-down, though the fishing, gambling folk dance their days away when the going is good.

Here Peters's fine Porgy, fierce on his crutches, falls hopefully and hopelessly for Hughes's imposing Bess, after her man, Cornell S John's swaggering, muscle-flaunting Crown, loses his temper, murders and vanishes.

Sam Douglas's white detective reminds you just how vicious institutional racism was. When Bess throws in her lot with Porgy, Peters breaks out in a poignant, amazed exuberance and song. Bess smoulders and broods eye-catchingly, almost swooning over John's pugnaciously charismatic Crown after he pops up on Kittiwah and reclaims her as his possession.

Back home, where Peters's sad, cuckolded Porgy reacts to Bess's confession with forgiveness and visits revenge upon Crown, Hughes seems a touch too wholesome and demure. She is not that convincingly tempted by the offer of angel- dust from O-T Fagbenle's admirably sinister Sporting Life, a cool, athletically gyrating dancer.

That is a minor complaint. The final image of Peters's devastated Porgy swinging his crutches as he sets off on his impossible mission to find the vanished Bess brings the musical to a finale of stinging pathos.

Some commentators nowadays write off this musical as dated, white, middleclass condescension. I disagree. Tony Kushner's Caroline, Or Change at the National, which analogously deals with oppressed blacks in the American Sixties, strikes me as lacking the power and glory of Nunn's Porgy And Bess.