Less than 18 months ago, Kate Nash was working in River Island. Now she's back on the high street, this time drawing hundreds of youngsters to HMV to catch a glimpse of her singing just five songs and signing CDs for 15 minutes.

Harrow girl Nash, only one month out of her teens, is the beaming beneficiary of a pop system that is moving ever faster.

In pictures: Kate Nash plays HMV Oxford Street

She surprised even herself with the performance of her debut single, Foundations, still perched in the number two spot after seven weeks. So much so that her album had to be rushed out almost two months early yesterday.

This instore performance was kept short so that she could sprint off to make a television appearance, but all the work seems to be worth it.

Her album, Made of Bricks, looks certain to make it to number one on Sunday, and the reams of fans clutching their copies in hope of an autograph seemed sure that the former shop girl is a star.

One teenager had written "I (heart) Kate Nash" on her cheek. Many others already knew every word of her songs, thanks to the popularity of the demos she posted on her MySpace site, now played almost four million times.

Paparazzi flashes must still be new to her, but she did her best not to look dazzled when she took her place behind her keyboard. In a red party dress, with a lime green teapot perched in front of her, she was invisible behind a wall of photographers throughout her first song.

That opener, Mariella, demanded the most work from her three-piece band of bass, drums and violin. It was a slowbuilding tale of an eccentric girl, though Nash seems keen to play up her own normality. She found her mother in the audience and said, "I'm sorry we had a fight this morning."

Mouthwash, her next single, rejoiced in the banality of everyday life, even containing the almost laughably low-key line: "I've got a family and I drink cups of tea".

Her songs were pure pop with the slight-est edge, all jaunty piano, glottal stops and swearwords, which inevitably leads to Lily Allen comparisons. Is she better than Lily? "Definitely," said fan Holly Martin. "I like them both," said Dan Cook. "Why should they have to be rivals?" Either way, there's yet another colourful woman at the top of the charts, and as the hordes here recognised, that's something to cheer about.