Britney Spears, 02, London

It's a measure of how fast the pop merry-go-round turns that the fairground rides on the forecourt of London's 02 are gently turning to the sound of Lily Allen's "Smile". Allen was 13 years old when Britney Spears first put all thoughts of homework aside with her unforgettable "... Baby One More Time" video. La Roux's Elly Jackson had not yet started secondary school.

But the original pop princess is back in town, kicking off the European leg of her Circus tour with eight nights in a big top that appears to have been designed with this very spectacular in mind. And spectacular the opening night undoubtedly is.

The fun starts with a turn by performers from the Big Apple Circus: essentially a little person acting as MC while strongmen and clowns, who look like Aqua rejects on Heelys, entertain the audience on the two small circular stages set up for such shenanigans.

The crowd, largely made up of girls who have grown up with Spears plus a dedicated camp following, whoop with delight. There can't have been this many bottle blondes in gladiator sandals in one place since first-round auditions for Caligula extras. The tight-T-shirt brigade has customised its outfits ("Britney's Bitches" reads one group's effort) for the occasion.

The troubles of tonight's main attraction have been well documented. Suffice it to say that since she last toured the UK, there have been marriages, meltdowns, two children, a bitter custody battle and the most publicised haircut since Samson. Such thoughts are swept aside as Spears descends, to the title track of her current album, The Circus, dressed in ringleader red and cracking a whip.

From about nine rows back, it's immediately clear that she is miming. Never mind, the stage is a blur of tumblers, ninjas, jugglers and dancers. The circular overhead video screens burst into life and the brilliant "Piece of Me" is performed with aplomb; its spot-on chorus ("I'm Mrs Lifestyles of the Rich and Famous/I'm Mrs 'Oh my god that Britney's shameless' ") second-guessing any reservations about lack of authenticity.

The show is apparently split into sections: "The Circus", "House of Fun", "Freakshow/Peepshow" and "Electro Circ". Such distinctions are hard to spot, and in reality the whole thing looks more like an extended advert for Agent Provocateur than a pop show you could take your kids to. The format does, however, allow for multiple costume changes. We get belly-dancer Brit, policewoman Brit, and any number of bikini-clad Brits. Act two ends on a high of sorts: current single "If You Seek Amy" (chorus: "All of the boys and all of the girls are begging to IF YOU SEEK AMY"), its lack of grammar hammering home the double-meaning (say it quickly) with all the sledgehammer subtlety of a Carry On film at its most crass.

Then, the shock of the night, as Spears takes off her Captain Scarlet headpiece and picks up a good old-fashioned microphone. "What's up, London?" she hollers before sitting in a floating umbrella to sing, not mime, the ballad "Everytime". The moment's potency serves to highlight how anodyne much of the music has been to this point. It's a startling reminder of the undeniable fact that, in spite of everything, Britney's got talent.

But the brutal truth is that for most of the evening a dancer could put on a blonde wig and perform the show while Spears sits backstage babysitting her children. No one would know the difference. As the closing triple-whammy of "Toxic", "... Baby One More Time" and "Womanizer" finally gets the crowd on its feet, we are left to reflect on how pop princess has become pop product. Her jingle? Lip-synching, whip-crackin', perv-teasin', girly-pleasin' ... Britney.

The Circus Starring Britney Spears tour has been a huge success in the States and looks set to conquer Europe. Its concept, at least, is genius, because when the theme of an evening is illusion, it allows a frightened 27-year-old to hide herself (and her many natural gifts) behind an arena-full of smoke and mirrors.

On selected dates until 14 Jun