It's easy enough to understand the immense popularity of acts such as James Blunt, Coldplay and Jack Johnson, the Hawaiian singer-songwriter who won a Brit and hit the number-one spot last week, a year after the release of his third album. They all make music with the edges sanded off, capable of reaching the largest possible audience through sheer inoffensiveness.

What is harder to comprehend, however, is the ear-melting level of crowd hysteria that met Johnson throughout his two London concerts this weekend. There was a huge contrast between the serene mellowness of his performance and the cacophonous squeals that accompanied his every barefoot toe tap. His collection of light, breezy singalongs has attained near religious significance for thousands.

Some of this mania might be explained by the fact that in the five years since the release of his debut album, Johnson has rarely visited London, preferring to play in places where he can engage in his first love, surfing. Most of the audience would have been seeing him for the first time.

Yet the majority also seemed to be longterm fans, converted before this year's first flush of UK success. They sang along fervently to older songs such as Taylor and the uncharacteristically upbeat Bubble Toes.

Johnson, the calm eye of the storm, did little to merit the obsession other than standing still, plucking an acoustic guitar and playing his songs pretty much as they sound on his albums. He did sneak a verse from the Cars' 1979 hit Just What I Needed into Sitting, Wishing, Waiting, covered the White Stripes' Doorbell efficiently and performed Damian Marley's Welcome to Jamrock with help from the man himself, but mostly it was left to pianist Zach Gill to provide any onstage movement.

Nevertheless, the singer was quietly in good humour throughout, gently leading the crowd through this large-scale campfire sing-song. He may have been so laidback he could have performed the entire set from a hammock, and he seemed as confused by the intensity of his adoration as I was. But if neither of us can grasp what it is, this placid charmer is clearly doing something very right.