Rodrigo y Gabriela, Hammersmith Apollo, London

For those about to rock, we salute you!" AC/DC scream over the PA, right at the end.

Moments before, Gabriela Quintero is really sweating, happily draining the last drops of energy from herself, hair going lank as she almost headbangs. One last acoustic chord is left hanging like feedback on the stage, while the stadium roar of AC/DC's audience merges with the more refined fervour of Rodrigo y Gabriela's fans right here.

The duo's defiant hybrid can be glimpsed then. They are refugees from Mexico City's metal scene, who literally abandoned electric guitars to apply equal aggressive virtuosity to the acoustic ones they dragged round beaches and hotel bars, playing for change from Mexico to their second home, Dublin. They have absorbed influences voraciously. The title track of their new, sixth album, 11:11, pays tribute to 11 musicians ranging from thrash- metal guitarists to Palestinian oud trios, with a large helping of jazz. Pre-Columbian art and religion from Mexico blurs into Hindu equivalents in their written explanations of their wholly instrumental tracks. But still, they're best known for their cover versions of Led Zeppelin's "Stairway to Heaven" and Metallica's "Orion".

The real mystery of Rodrigo y Gabriela is that they have taken the sort of virtuosity despised outside of metal since the punk schism of 1976, thrashed it out on acoustic guitars favoured by sensitive singer-songwriters, and gained a growing cult following a small nudge from the real mainstream. More prosaically, these are Mexican buskers who forged a reputation playing on Dublin's streets, and by willed daring now headline the Hammersmith Apollo. The whole affair finds me staring blankly at times, wondering if the punk wars were fought for this. But I'm also amazed once again at how huge sections of the public simply ignore what corporations and journalists tell them to like. There has been, as Rodrigo Sanchez complains, a "painful" lack of radio play here. And how exactly do you hype two hours of often improvised, acoustic instrumental Latin jazz-thrash guitar? But on a rainy, windswept night, which has filled this place to the rafters.

Rodrigo y Gabriela are growing with their cult success: where before they perched on stools, Sanchez now splays his legs like a rock star, while Quintero keeps a foot on the monitors, behind which lie the effects pedals which assist their extraordinary sounds. Theoretically a rhythm guitarist, she beats big, booming chords from her instrument. Her fingers flutter with such dexterity that when she switches to a darbuka (goblet drum), I'm momentarily sure she's strumming strings on its top. When she solos, wah-wah effects sound like weird birdsong or mouth-music. Towards the end of the epic "11:11" she digs out alien beeps and coos, electronically and imaginatively squeezing out everything she can find in the old stringed box of her acoustic guitar.

The alchemy between her and Sanchez is sometimes hidden as they turn fondly towards each other. His own solos switch from aggressive riffing that quotes the White Stripes' "7 Nation Army", to a mode where you can hear the breathy whistle and squeak of the strings. The weepy balladeering of a John Williams peeks out too. When Quintero rejoins him for "Anushka"'s Middle Eastern interlude, arcing blue spotlights also pick out a lower, sombre mood. But the pair are more likely to lightly pogo and encourage cheers from the crowd. For all their commercial oddness and creative curiosity, Rodrigo y Gabriela's secret is maybe quite simple. They are resourceful musicians who can really play, and open-hearted, happy entertainers. That, very often, is what people want.