When Os Mutantes' Sérgio Baptista comes on stage to fix his guitar during the sound check, he receives a standing ovation. Evidently, after a 33-year hiatus, just showing up is enough. Os Mutantes are that kind of band - they inspire obsessive devotion. But what sets aside this rock reunion from others is that they were never famous outside of their native Brazil, making tonight more of a debut than a comeback.
Like me, most of the audience are too young to remember them the first time round. Formed in mid-Sixties São Paolo by teenage brothers Sérgio and Arnaldo Baptista, and Arnaldo's girlfriend Rita Lee, they were part of the brilliant, bonkers and brief Tropicália movement. Their music was like nothing else - a marriage of Beatles psychedelia with hard rock and Brazilian rhythms. While fellow Tropicalistas Caetano Veloso and Gilberto Gil went on to fame and fortune (indeed, Gil is currently the Minister for Culture), Os Mutantes took too much acid, stopped speaking to each other and became period curios. But their influence spread far and wide. Kurt Cobain invited them to reunite as Nirvana's support act, but was turned down, and Beck named his 1998 album Mutations for them. This time round, Rita Lee couldn't be persuaded, but even so, the concert is a sell-out.
Marching on stage clad in tights, boots and capes, like Robin Hood's Merry Men, looking surprisingly sprightly and tanned, and with a sizeable band to back them up - including the tomboyish Zélia Duncan in Rita Lee's place - they are a force to be reckoned with.
"It's great to be here," says the perennially smiling Sérgio. His older brother Arnaldo cuts a more tortured figure, hunched over a Hammond, doing weird hand-signs to the songs. There isn't any notable tension between the two, but they stand at opposite sides of the stage, and communicate very little (although Sérgio does introduce Arnaldo as "my dear brother").
The band are more than just a bunch of session musicians: the backing singers pull faces at each other, Zélia prances around like a mad hippie with a tambourine, and at one point, someone plays a bass guitar like a cello. You could almost forget you were in the Barbican. The temperature shoots up and people pull their jumpers off and dance. If you didn't know any better, you'd see a band at their peak, playing for their lives.
All the hits are there. Mambo parody "Cantor de Mambo" gets the middle-aged music buffs on their feet. "El Justiciero" is introduced by some Bush'n'Blair baiting from Sérgio and prompts a spate of flamenco dancing on stage. When "I'm Sorry Baby" descends into a gospel-like euphoria, the concert turns into a revival meeting: the audience with arms aloft, smiling beatifically. And at the end of "I Feel a Little Spaced Out", Sérgio plays the freaky, prolonged guitar solo that I've been waiting to hear all of my life. On his knees. I throw aside my notebook, and join the worshipful throng. As do nouveau hippies Devendra Banhart and Noah Georgeson, who join the band on stage to sing along with the twisted samba of "Batmacumba".
Long after the lights have come up, the crowd are still chanting "Mutantes! Mutantes! Mutantes!" As a Brazilian, seeing an obscure South American psych-rock band receive such rapturous applause from a young London audience almost brings a tear to my eye.
In a way, Kurt Cobain was lucky that Os Mutantes turned down that support slot - Nirvana getting out-rocked by a bunch of mad, middle-aged Brazilian hippies would have been a sight to behold.