In the late evening sunshine, an a cappella version of Somewhere Over the Rainbow arcs over the warm Camden air. Inside a 1930s red brick community hall, 200 singers are warming up for their Monday practice. And, sitting in the garden beyond, Martin Brophy is explaining his choir’s particular difficulties in finding a venue.

“A few years back we were forced to try lots of places. I knew that the Sisters of the Immaculate Conception had a hall, and in desperation asked if we could book it,” he says. “This little Irish lady said [he puts on a Mrs Doyle accent]: ‘You’re called LGMC. Now what does that stand for?’ I replied: ‘We’re the the London Gay Men’s Chorus.’ She made me repeat it a few times and said: ‘I think I’m going to have to check this with the reverend mother.’ They eventually let us sing, as long as we signed an agreement that we wouldn’t have sex in the toilets.”

Luckily for his choir, and the 59 other choirs who will join him next weekend for Europe’s largest gay and lesbian choral festival, the Southbank Centre is less exacting. Various Voices, in the South Bank Centre, is a four-yearly, four-day celebration of one of the more unlikely byproducts of the gay rights campaign: homosexual choirs. From Out’n’Loud in Finland to Mélo’Men in France, the movement that began as a novelty act in 1980s pubs now encompasses choirs who sing to professional standard, and more than 10,000 members.

The spread is geographical, but it is also temporal — a slice through the social history of the gay rights movement. The Brighton Gay Men’s Chorus, who last year reached the semi-finals of Last Choir Standing, are unlikely to face homophobia on a daily basis. But for Le Zbor, from Croatia, every practice is an act of defiance. The newly formed choir is all-female, not through choice but because Zagreb’s male homosexuals do not yet feel ready to stand up and be counted. Short of funds, they are able to attend the festival only after Various Voices waived a registration fee and helped to raise money for their travel.

Even if most choirs function as social clubs as much as campaigning groups, almost all started like Le Zbor — small, poor and with a mission to bring about social change through the power of music. The movement began in the United States. In 1978 Harvey Milk, the openly gay supervisor of San Francisco, along with the mayor George Moscone, were assassinated. Their deaths became the rallying point for an appalled city, and inspired the first performance of a newly formed group: the San Francisco Gay Men’s Chorus.

A New York rival soon formed and rapidly reached a standard that allowed it to play Carnegie Hall. A gay choir producing such world-class music was a more powerful symbol for homosexual rights than a hundred rallies. But the rise of gay choirs coincided with the rise of HIV, and many found themselves performing at benefit gigs and at funerals, often for fellow members.

Britain took longer to join the movement. Robert Offord was one of the founder members of LGMC. “When we started in 1991 there was a lot of hostility to gay culture,” he says. “There was the debate over Clause 28 and Aids was well established – along with the suggestion that gay people were somehow responsible for its spread. There was work to be done and the best way we saw to do that was through music. There is a power in just standing up and saying: ‘I am gay, get over it’.”

His choir is now 200-strong, but at their first concert, busking in Angel Tube station, they had nine members. “There was a debate about what we should call ourselves. Rainbow Singers was too coy,” Offord says. “I painted the words Gay Men’s Choir on a sheet of paper. The name described what we were: nothing more was necessary, and nothing less acceptable. People’s jaws just dropped,” he recalls.

Now, 18 years later, they are mainstream enough to headline at one of Britain’s premier arts venues. Today gay choirs are more likely to sing at civil partnerships than at funerals of people who have died of AIDS. But what is the point of a gay choir? What do they provide that cannot be found in straight choirs (“non-gender-specific choirs,” Martin Brophy corrects me). Sophie Fuller, a teacher at Trinity College of Music, is running a workshop at the festival that attempts to answer that question.

It is titled: “Queer studies: How can music be gay?” “There is no kind of inherent essence of gayness in music,” she says. “But there are moments when music does seem, er . . .” she pauses, “. . . very gay. Kylie, Judy Garland, Girls Aloud, Abba — when a particular kind of gay man performs these there is such a wonderful element of campness. The gayness is in how the audience see it, an association with the drama and the tragedy of these wonderful divas.”

The repertoire of the choirs at Various Voices takes in medieval chant and classical music as well as more contemporary songs. But there is, undoubtedly, a certain emphasis on show tunes. Diversity, a mixed lesbian and gay London choir, once themed a concert around South Pacific in which the parts were reversed. So the men sang I’m Gonna Wash that Man Right Out of my Hair, and the women responded with There is Nothing Like a Dame.

“We are political from the moment we step on stage, but we are not confrontational,” Alisdair Low, the vice-chairman of LGMC, explains. “In more oppressive societies, bars and clubs might be raided. But no one raids a music meeting.” Then there is the social element. “It is an easy way to meet other gay people, without the overtures of sexuality,” Low says. Later he recounts a less on-message tale about a recent choir tour, involving a hotel swimming pool, a singer who was into S&M, a dog lead and some rather shocked German tourists.

There must be the odd romantic entanglement? “I thought it would be a massive kinky shagfest,” says Chris O’Neill, one of the newer recruits, “smouldering looks over the choir books and all that, but it’s not. It’s more about being friendly. This festival will be about . . .” O’Neill pauses, then jokes: “Oh, sod it, it’s going to be an international shagathon.”

Various Voices, Southbank Centre, May 1-4, www.variousvoices.com

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