A Silver Mt Zion, Koko, London

As the cornerstone of the Canadian label Constellation Records and the heirs to Godspeed You! Black Emperor, A Silver Mt Zion are the most mysterious of bands. The mythology would have you believe they are black-bloc anarchists, that they record in disused warehouses populated by portentous ravens, and that they don't do interviews because what they have to say may kill you.

The truth of the matter is that A Silver Mt Zion are a severe and radical force, the long-lost conscience of independent music. In north London tonight, they are like avant-garde missionaries, pushing wit, emotion, and 15-minute experimental rock songs on the wide-eyed heathens. Two violins, two guitars, a double bass, cello and drum kit grace the stage throughout, and out of them comes a beautifully formed hybrid of soaring anthems, heart-rending orchestral laments, and arrangements reminiscent of free jazz.

Their second song, "Take These Hands and Throw Them in the River", typifies the band's ambition. It begins with an apocalyptic, violin-heavy crescendo, essentially taking the point where a traditional rock song finishes as a starting point. This creative approach to verse-chorus-verse formalities illustrates what the ill-loved but often-used genre name "post-rock" actually means.

They play seven songs in two hours, but the evening flies by. "One Million Died to Make This Sound" starts with three minutes of enrapturing choral exchanges between the septet, before kicking in and morphing into a rock elegy of grandiose proportions. What the elegy is for is not clear, but lead singer Efrim Menuck's gruff yet mournful vocal style has a way of making one want to weep for the world at large.

Elsewhere, thesound of all seven musicians wailing: "God bless our dead marines" a cappella at the end of 10 minutes of impressionist rock left a 1,000-strong crowd numbly, humbly silent.

"American Motor Over Smoldered Field" is introduced as a song "written about the American President when he was still just an horrific novelty". When an Asbo joke - from a Canadian band - is met with a mixture of laughter and consternation, Efrim explains self-effacingly: "We're not like other bands - we do research." This is not the only thing marking A Silver Mt Zion out from their peers.