Oasis, Heaton Park, Manchester

For three opening minutes, Manchester's cockiest band are hugely impressive. Their dense sound is crushing, even across one of the city's larger green spaces, with touches of finesse from a group firmly in control. Liam Gallagher's snarl, too, is at its most devastating.

Then the sound cuts out for an instant, the crowd filling in on "Rock'n' Roll Star" regardless. When it drops completely, they finish the line and only fall silent themselves as they realise that something is seriously wrong. Oasis return briefly, only for "Lyla" to be cut short, then they disappear again for a worrying half hour. Smoke billows from a blown generator. Will they come back? Rumours from the hospitality tent suggest they have already fled.

Of course they return, rather than disappoint 70,000 fans. "This is a free gig from now on," the singer exclaims. "You'll get your tickets back." "We're gonna play 'til they throw us out," elder brother Noel adds. Suddenly, what would have been another notch on Oasis's relentless global march (they set out last August) has the potential to turn, with perfect timing, into a special, one-off event. In a recent interview, former guitarist Bonehead suggested that they should have split up after the mammoth Knebworth events, for they could not be surpassed, at least in terms of scale.

Such musings from an ex-band member would count for little unless many people could be forgiven for thinking similar thoughts, including the group themselves. This time last month, Noel was in South America complaining that Oasis were "a rudderless ship". Liam, while countering his claims, was concerned with his clothing label Pretty Green (named after the Jam song). The label launched the very day that the band kicked off a UK stadium tour with three dates in their home city. Tonight he is wearing his regular Mod-style anorak – but this time adorned with the range's vaguely psychedelic logo.

So it takes a power failure to knock this well-oiled machine out of its comfort zone, and for much of the time, Oasis rise to the challenge. "Last time I was here was to see the Pope," Liam marvels. Now the crowds have flocked to see him mainly stand motionless for two hours. It would have been a hit-filled set anyway, but the singer applies extra urgency to the old standards, spoiled occasionally by a peculiar barking affectation at random intervals. Only a limp "Wonderwall" sounds tired.

Noel's turns on vocals, meanwhile, have become integral parts of an Oasis set. He brings a subtle melancholy to the downbeat anthem "The Masterplan", a throwback to when Oasis B-sides were worth collecting. It is noteworthy that no one resorts to raising a lighter or mobile phone; rather, they join in full-bloodedly with the words. Liam fares less well on the self-penned "I'm Outta Time", his snarl losing the rare vulnerability he shows on the recorded version. This is one of the more intriguing moments on the current album, Dig Out Your Soul, and suggests the band are reaching for more emotional depth. The krautrock groove of "The Shock of the Lightning" restarts the set and holds its own against the group's hedonistic classics.

It's on less memorable numbers from the album that the band let slip how dependent they are on others' good ideas. "Waiting for the Rapture" evinces a fondness for the Doors' "Five to One", while with its dull retread of the drums from "Tomorrow Never Knows", "Falling Down" spoils the home straight of a gig that Oasis perform without pausing for an encore. Nevertheless, the band somehow break their curfew by a generous 30 minutes. Noel might regret the money-back offer, but as another of his songs suggests, no one need look back in anger.

Touring to 12 July (0870 534 4444; www.myspace.com/oasis)