"I wanna rock the microphone now, without the panty hose," shouts Wilco frontman Jeff Tweedy, before grinning ironically. "I'm taking frontman lessons," he explains.

Tweedy may need them if Wilco's commercial stock rises as high as their critical reputation. The Chicago band practically invented alt.country in their earlier incarnation as Uncle Tupelo; their last two albums, 2002's Yankee Hotel Foxtrot and this year's A Ghost is Born are strange, compelling collections of country and avant-rock.

It's cheering to see a leftfield band on such crowd-pleasing form. After playing Hummingbird, Tweedy makes a throwaway remark about it being a good song to jiggle about to. "Play it again, Jeff," comes a voice from the throng, and the band, without a whiff of hesitation, do, while the crowd bounce around.

Experimentalism isn't off the menu, though. Wilco's songs mostly start like punk rock charges or country laments, but many duck into post-rock territory - the band adopting fierce expressions while playing daunting, complex music that tears away at the PA system. Guitarist Nels Kline (brought in alongside keyboard player Pat Sansone to replace multi-instrumentalist Leroy Bach) plays Hawaiian and jabs his instrument with an assortment of toys.

The effect is sometimes a little sludgy, but when it works, this structured mayhem is stunning. As its roar abates, and Tweedy's delicate, wry narratives about love, drugs and the devil re-emerge, they seem all the more fragile and resonant.

Encores I'm a Wheel and The Lonely One offer a fine illustration of Wilco's range: the former, played at almost double time to beat the curfew, is a rambunctious rocker, while the latter closes the night with a self-conscious, bittersweet air. The crowd cheer with such fervour that you half believe the band will be back on, despite the shine of the house lights. Tweedy may not need those lessons after all.

· Wilco play the Astoria, London WC2, tonight. Box office: 0870 1544 040.