Taraf de Haidouks, Royal Festival Hall, London ****

One moment, the stage was bare; the next moment, Taraf de Haidouks were beating up a storm, with layer upon layer of exhilarating sound: flute over violin over clarinet over accordion over cimbalom over bass. The Gypsy band from the Romanian village of Clejani have come a long way since their discovery 10 years ago by two young Belgians (hence the "de" in their name). Taraf means band, and haidouks are brigands: there are many other tarafs to be found in their part of the world, but this bunch live up to their name. They spent the Nineties wowing Europe and America with tours and CDs, and last year they set the seal on that success by winning a BBC Radio 3 award, and by getting their fanatical admirer, Johnny Depp, to shell out a fortune for benefit concerts in LA.

The South Bank audience were fanatics, too, but so are we all: once heard, forever smitten. This art draws on multiple roots, from Hungary, Turkey, and the Arabic world, as well as from north India, whence the Gypsies originally came. The Haidouks' style resides in the contrast between a strictly repeated bass line and dazzling improvisation above; their fast numbers are offset by slow laments using a limpingly irregular rhythm. Until fame struck, they earned their bread the hard way, playing at wedding feasts that could last three days, and that village ambiance is what has made them irresistible to urbanites in the West.

At the Festival Hall, after they had whirled through a Turkish dance, a grey-suited gent delivered a ballad with charmingly old-world courtesy. Then the players took it in turns to show off their virtuosity: Paganini-stuff on the violins, pyrotechnics on the accordions, and Chopinesque cadenzas on the humble hammered zither. Once the audience twigged that the performers wanted them to dance – to hell with health-and- safety regulations – the whole place became a dance hall; this sober building can seldom have seen such a joyful riot.

But as the evening went on, it became clear that something had changed, something was missing, quite apart from the late lamented Nicolae Neacsu, with his endearing stunts on a trailing string. We waited in vain for the wide vibrato and piercing pathos of the slow songs, we got no whiff of the village from which they sprang. The Haidouks are now superstars, and their well-polished act, with its nifty bits of clowning, has become predictable. Those who love their quirkiness, their sheer otherworldliness, will have to go back to the fabulous CDs that they made in the Nineties for the Craw label, because those qualities may never grace the stage again.

Taraf de Haidouks play Warwick Arts Centre, Coventry, tonight, and Vicar Street, Dublin, tomorrow