REM, Brixton Academy, London

Seeing a band of REM's stature at a medium-sized venue such as Brixton Academy is undeniably some sort of occasion. The suspicion nags, though, that at this gently sliding stage of their career, such a back-to-the-roots ploy may soon be forced on them anyway. REM have been minus a founding member since the departure of the drummer, Bill Berry, and jettisoning fans since Automatic for the People, now a full decade ago - it's sometimes hard to remember why they seemed so important for so long. Never attached to any social or musical movement, never about anything more than themselves, they have never dated. But, by the same token, they have never attracted devotion like bands who have managed to define eras. REM's uniqueness has consisted simply of staying good for so long - even with their recent, less successful albums. But when people stop listening, does that really matter?

Suddenly, Michael Stipe is on stage, just yards away, whipping off a Stetson, prancing in shades and a white suit, and singing "Get Up" from 1989, the year before their giant mainstream success. Peter Buck, the band's most unreconstructed rock musician, moves pleasurably, almost in slow motion, at Stipe's side. Passing swiftly through a rattling, unrecorded song, the singer is soon spinning jerkily into "The Great Beyond", REM's best recent single. The reaction in the crowd says a lot. People are punching the air in a way that is far short of ecstatic. They look puzzled, even dazed, as if they only half-remember the past decade of REM as stadium rockers, and know that this is their natural place.

When Stipe reminisces about learning from The Smiths, who split up before moving into the mainstream, you're reminded what a historical accident REM's success is. Then, when they career through the fans' favourite "The One I Love", you realise they deserve it, too.

Stipe's stagecraft is certainly exceptional: at one point, with one hand on his mic-stand, he extends both legs in mid-air. He has learnt from David Bowie as well as Morrissey. "Electrolite" and "At My Most Beautiful", both from the past seven years, show that the band have not lost their touch at devastatingly lovely ballads, either.

A new song, "Bad Day", suggests that their next album will be a denser, more rocking affair than recent offerings. Then comes "Losing My Religion", their breakthrough single, which inspires a mass sing-along and huge cheers. "Man on the Moon", their best song of all, gets a genuinely passionate response, not least from Stipe. Flashing across the stage as though shot from a gun, impersonating first Elvis then a bounding rabbit, and then simply howling straight at us, at this instant he matches his legend. Stipe looks ecstatic as he waves goodbye.

Then comes the first encore: "Everybody Hurts". If we were in a stadium, lighters would be lifted at this moment, and people around me do seem moved. But there is little hurt, or love, in Stipe's singing. And it begins to feel that this night as a whole is just a little too late in the day, for band and fans - that nobody can quite manage to raise the enthusiasm they used to.

This gulf between intent and effect yawns widest at the very end, when Stipe squirms and declares himself, apologetically, ashamed of being American. He makes a personal, improvised pledge of allegiance, revoking the one to his President, while Buck jerks a hostile squall from his guitar. It is a moment meant to tie REM and their fans to Hendrix's demolition of "The Star-Spangled Banner" during Vietnam, and the anti-Bush Snr protests of "Drive" (with its "bushwacked" jibes) and "Ignoreland" a decade ago. But the crowd, invited to join in a moment of bonding that is about more than nostalgia, simply stare - not comprehending, or not caring. "It's the End of the World as We Know It" finishes things, the irony of its title, and the crowd's renewed, happy cheers, lost in the general emptiness.

REM headline the Pyramid Stage at Glastonbury tomorrow night