Fancy an international 600-strong sing-song on moving staircases?

Under the protective clustering of a rather macabre clump of antlers, collected together in the bowels of Cecil Sharp House (the London headquarters of the English Folk Song Society), Jeremy Haneman, director of the London Gay Men's Chorus, is putting some 75 men through their paces. At his command, they "ooh", "ahh", massage each other's shoulders and, warm-ups complete, launch into the emphatic refrain that runs through one of the most unusual and exhilarating musical events to be performed in London in recent years.

Under the protective clustering of a rather macabre clump of antlers, collected together in the bowels of Cecil Sharp House (the London headquarters of the English Folk Song Society), Jeremy Haneman, director of the London Gay Men's Chorus, is putting some 75 men through their paces. At his command, they "ooh", "ahh", massage each other's shoulders and, warm-ups complete, launch into the emphatic refrain that runs through one of the most unusual and exhilarating musical events to be performed in London in recent years.

"Why do you sing?" carols Haneman, rehearsing the part that, at the two Roundhouse performances of Because I Sing by Belgian director Alain Platel, will be sung by The Shout, a 20-strong choir led by composer Orlando Gough and musical director Richard Chew. "I sing because I sing", the men chorus back, their harmonies already threading a narrative of their own. "I sing because I sing."

But why – and when – do people sing? Modern life offers few chances. Church attendance is in freefall and the secular opportunities – among football crowds, perhaps – are limited. Yet anyone involved with music at any level can testify to the emotional surge that singing – being in unison with others – offers. Conductor Ben Zander uses song in his motivational seminars by getting his classes to sing (and they do, lustily, and in German) Beethoven's "Ode to Joy". Healing centres now offer singing groups to the world weary.

It's these questions that occupy Platel most. Written specifically with the Roundhouse's acoustics in mind and involving some 16 choirs, in addition to The Shout, and 600 singers, Because I Sing has as its aim the mapping of a songline for London out of the capital's diverse communities. "In all our conversations with the choirs, we asked what made them sing," says Platel. "We never found an exact answer. Some hide behind God and talk about spreading a message. Others describe it as a feeling of power, to be able to communicate. One thing everyone had in common was once they started to talk about singing, their behaviour changed. They become very excited."

This should give some sort of clue to what Platel (who rose to international attention in 1984 via a series of award-winning productions with Belgium's Les Ballets C de la B) has in mind for the Roundhouse. A director who once installed a Dodgem track in the venue for Bernadetje in 1996, he's interested in the tension between the power that masses of people evoke and the fragility of the individual. The latter is highlighted by a portrait exhibition, by no means ancillary to the main event, featuring every singer involved. While the singers themselves will be static, the 600 will be crammed in every nook and cranny of the Roundhouse and giant moving staircases will change the emphasis of each section.

"Choosing the choirs was a problem", says Platel. "We had three times more than we could take. Orlando, Sophie [Fiennes, who is filming the entire process for Channel 4], Michael [Morris, the producer] and I all talked with them about why they do what they do. But we didn't choose the best. We chose different choirs. Some of them are extremely good, with a level that's professionally high. Others are very fragile, but by just being together they show something very particular. The Swiss Church Music School Choir, for example, is made up of refugees from Rwanda and elsewhere. They have a Swiss leader, and that's it."

And, as is to be expected with a cast of 600, the inconsistencies multiply. Suddenly the choristers aren't all that they seem. The London Diocese Deaf Choir don't sing, but sign. There's a Scotsman in the Ngati Ranana Maori Choir. and none of the members of west London's Maspindzeli Church choir are actually from Georgia: they just happen to be singers dedicated to Georgian polyphony. Moreover, there's nothing in the constitution of the LGMC that says members have to either gay or male.

These flexible memberships and cross-cultural permeability form part of the Roundhouse events' narrative cohesion, suggests Platel. Music has, in his view, a function that transcends the notions of performance bound by time and place. He recalls how he stumbled across the medieval theory of humours – a system in which human wellbeing, both physical and spiritual, was governed by the interaction of fluids contained in the body itself.

"When I worked on Bach and Purcell projects, I heard of this theory, still influential in their period, about how an excess of one fluid or another would make you ill. Melancholy, for example, was thought to be the result of too much black bile. Because it was believed that art in general and music in particular could have a therapeutic effect, many composers at the time were trying to make music where people could cry: weeping would tap off the excess fluid. It's a very naive idea, but I love it."

'Because I Sing': Roundhouse, NW1 (020 7771 2000), Saturday, 1 April. 'From the Ritz to the Anchor & Crown' by the London Gay Men's Chorus is released by Dress Circle

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