"There can doubtless be no mistake about the genre of this work," wrote Berlioz of his Romeo et Juliette. "It is neither a concert opera nor a cantata, but a choral symphony." That's hard to take at face value. A lot of the work is powerfully symphonic, but there's plenty that seems to come from other musical worlds. The finale could easily be a chunk of a grand opera; "Romeo at the tomb" is vividly pictorial programme music (with the clarinet "acting" Juliet's awakening); and the Strophe from the First Prologue comes close to sung literary criticism. But try separating the purely symphonic element and you'd soon run into trouble. The strands are seamlessly interwoven. Better accept Romeo as the unique, unclassifiable thing it is, and revel in it.

Judging from his performance at the Theatre Royal, Drury Lane, last Sunday, that would seem to be John Eliot Gardiner's view of Romeo too. Despite the interval between the "Queen Mab" Scherzo and the newly restored Second Prologue, the whole thing had a magnificent Berliozian sweep, not often matched in concert or on record. And yes, the Second Prologue (idiomatically orchestrated from Berlioz's surviving piano score by Oliver Knussen) does add to the overall balance and narrative logic.

But Gardiner played up the theatrical elements too. The chorus filed off slowly at the end of "Juliet's funeral procession", and the music sounded as though it had been written specially to accompany them. Six harps took centre-stage for the "Concert and Ball" - delightful to be able to see them working so hard, and even better to be able to follow their brilliant writing so clearly. In the finale, the Montague and Capulet singers wore different coloured blouses - a modest device theatrically speaking, but it made sense to see the dramatic opposition as well as to hear it.

The Theatre Royal has strong links with Berlioz (he conducted his music there more than once). But acoustically there were problems. Set back on the stage, the orchestra sometimes felt remote, especially in the finale, with all the choruses placed at the front. The woodwind sometimes sounded muffled, with the soloists apparently straining to establish a proper presence. The drama of "Romeo at the tomb" suffered, and not only in that "awakening" clarinet solo (Berlioz marks it pppp, but he obviously expected it to carry); the orchestra outburst ("Delirious joy") that follows was surprisingly short on passion or textural focus.

But that was exceptional. Even in these less than ideal conditions there was plenty to enjoy in the playing of Gardiner's period instrument Orchestre Revolutionnaire et Romantique. Tone and intonation sometimes coarsened in the heat of the moment, but that added an expressive edge - a sense of danger that super-accomplished modern orchestras often lack. The virtuoso orchestral effects of "Queen Mab" were effectively born again: eerie violin harmonics, a gorgeously plaintive cor anglais and distant horns that could have been calling from some haunted woodland.

One shouldn't forget the singers. Bass-baritone Gilles Cachemaille (Glyndebourne's recent Don Giovanni) and the Monteverdi Choir made gripping drama of the finale, reaching an overwhelming and thoroughly convincing climax in the colossal waltz-tune at the end. It was Mahler who said that a symphony "must embrace the world", but he never managed to cram more into a work, with more assurance, than Berlioz does in Romeo et Juliette.