The "Arts of Armenia" festival had been presenting exhibitions and films in some of London's livelier spaces for a fortnight. When it came to the finale last Monday night, the entire directory of Armenian residents seemed to be trying to get into the Queen Elizabeth Hall. They ensured a lively reception for the Academic State Choir of Armenia, making its first high-profiled UK appearance - a nightmare of a name, but a definite national treasure.

We used to hear a fair amount of Armenian music here in the days when the Shah of Iran promoted it as part of his Westernisation policy. That was usually early or contemporary music, in the hands of emigres. The State Choir is a Soviet product, formed in 1936 and directed for the last 33 years by Ohannes Tchekidjian, a Turkish- born musician who moved to Yerevan to take up the position.

The repertoire it brought was religious and traditional, arranged in romantic and 20th-century styles, all unfamiliar and absorbing. But the choir itself was the main attraction. From the start it delivered a warm, rich-toned sound of astonishing force and dynamic flexibility - vibrato heavy but well synchronised, and dispensable in the lighter numbers where speed was all. Most of the music was unaccompanied, with a few items bringing on an unnamed pianist with luxuriant black hair and gaunt profile. Everything was performed from memory.

This meant a virtuoso display of interpretative sharpness. Tchekidjian roamed restlessly in front of the choir to make fierce or caressing gestures, well ahead of the beat, with a pair of hands that combined sensitivity and decisiveness. The audience couldn't take its eyes off them. More to the point, neither could the choir, who were able to sing with minute freedom of tempo and extreme delicacy of nuance. Their specialities included quick turn-rounds of pace and fierce swellings of tone on sustained chords.

Soloists of imposing power would step forward from time to time. The whole choir, 48 in number, could make the hall resound in a way that we are used to here only with groups twice the size. Tenors seemed subdued at first, but this was not the standard problem of choirs having to be grateful for any tenor they can get to join them - rather, a matter of high-lying lines, negotiated with accuracy. Later, some unsuspected blasts of sound emerged from them. Low basses you expect in groups from this source; the precise placing of quiet notes below the massed harmony, like organ pedals, was a new experience.

After the soulful surge of the first half's liturgical extracts, folk- song arrangements by Komitas brought a contrasting breeze of vigour and asperity. The Hallelujah Chorus, their encore, will never sound the same again: mighty fortes and sudden hushes led to a drawn-out cadence of a lung power that in the English choral tradition simply does not exist.