World Voice, South Bank Centre, London

It's wonderful to be reminded, as we breed our young couch-potatoes, just how clever children can be. The latest incarnation of Finland's award-winning Tapiola Choir has come to the South Bank as part of the WorldVoice festival, and their spell is as bright as ever. They have a conductor, but he hardly appears at all: these bright-eyed kids aged from eight to 18 manage the most complex feats of part-singing as though nothing could be more natural.

Their voices in their opening song were fresh and unforced, but their attack was spot-on; then, in a series of songs created for them by contemporary Finnish composers, they showed what they could do. They hardened their tone in the ancient peasant style, till it had the impermeability of pebbles on the beach; they delivered a song that was all groans, cries, and susurrations. They showed how a phrase could seem to pass from one end of the stage to another.

Then - and I've never been so charmingly inveigled into audience-participation - they roped in the whole of the Queen Elizabeth Hall for a piece entitled "Eternal Echo". Three simple motifs were first sung from all four corners of the auditorium, then enriched layer upon layer, until the whole hall was awash with cooing sounds. Finally they offered a medley of songs and dances, creating their own instrumental accompaniment with violin, clarinet, and drum. There may be as much talent in our own choirs, but there's nothing like Tapiola's technical adventurousness.

If this was all that was on offer, we'd have had a great evening, but it was part of a double bill: after the interval the Rossica Chamber Choir sang Rachmaninov's "Vespers". This has become a popular concert work, but this choir from St Petersburg took us back to the rawness of its roots. They had a priest who functioned as cantor, and a lady conductor who gave pitch-cues, as is done in Russian churches.

The Russian Orthodox liturgy is the purest expression of the Russian soul: Rachmaninov's majestic work is a celebration of its musical richness. And as Rossica showed, it's about voices pure and simple: from basso profundo, to dark-velvet mezzo, to high sweet tenor, to soaring soprano; their baritone soloist had a voice of heart-stopping beauty. This a capella music is orchestral and operatic to the nth degree, you've got to feel in your chest its journey over peaks and chasms, and its vocal trumpet-blasts. If Rossica were to take up liturgical residence in London, the church would be packed every night.

The WorldVoice Festival, to 12 October (0207-960 4242; www.rfh.org.uk)