Britney Spears, O2 Arena, London

After 15 minutes of wretched circus capers, involving a dwarf and a burly man spinning a cube, Spears finally emerges, suspended in a glittery oval ball, performing underneath what looks like a giant lampshade from Homebase. A small wave of faithful, glitter-drenched Britney clones howl their approval. It's not exactly deafening appreciation, but there's a lot of forgiveness and, well, love, in this room as the majority wilfully stomach the obvious lip-synching, tatty magic tricks (Paul Daniels would be ashamed), muddled choreography and lack of engagement from the main attraction. But, still, this is pop royalty and she's owed a bit of slack.

Has anyone, in fact, been so ceaselessly hounded by the press pack as this fragile 27-year-old Southern Baptist from Kentwood, Louisiana? Divas such as Beyonce, Pink, Christina Aguilera are pitched as survivors, enforcers, warriors even, but this former Mickey Mouse Club member and indisputable pop superstar – she's the eighth bestselling female recording artist in US history – always comes across as the victim ("my loneliness is killing me", indeed), a Hitchcock blonde regurgitated on to the tabloid pages, all in a blur of unhinged activity.

A roll-call of this low-rent Marilyn Monroe's mishaps reads like a Davina McCall-style final farewell to a Big Brother contestant, here's your "best/worst moments in the house, Britney"... frequently knickerless, shaving your hair off with clippers, smooching Madonna, splitting with Justin Timberlake, a marriage that lasted 55 hours, custody battles, driving hiccups and reams and reams of disturbing images...

This multi-millionaire "victim" has peeved everyone off from the American Family Association to PETA. But, like Princess Di, no one seems to get enough of her and that's why her second number tonight, the mischievous, "A Piece of Me" is such a sensational, defiant electro-pop song; an anthem for the persecuted, delivered with righteous venom on her best album, 2007's Blackout, produced by Nate Danja Hills. It's the stand-out number tonight and the naughty, wonderfully self-aware opening lines – "I'm Miss American Dream since I was 17/Don't matter if I step on the scene or sneak away to the Philippines/They still goin' put pictures of my derriere in the magazine." – is a teeny thrill.

The rest of the set – which is divided into five pretty mind-numbing themed sections – Circus, House of Fun, Freakshow/Peep Show, Electro Chic and, er, Encore, but they all could just come under the umbrella title of Bordello – is gaudy, seedy, quite often nonsensical and not quite bonkers enough, with the obligatory (also see Madonna, Kylie) accompaniment of bondage-clad, pecs-out male dancers worshipping, gyrating and preening around a clunky, oddly stilted Brit, who endlessly changes into a variety of differently coloured bras and pants. In fact, she spends most of the time changing into different garbs; not really, you know, singing. It's panto or Vegas. Or both.

The gutter press will obsess over her weight; but the most worrying thing on display here is the degree to which she might be lip-syching and her weaker efforts, queasy electro-pop dirges, in particular "If You Seek Any" and "Circus" on her latest album, Circus. She's clearly in a happier place right now, but the new tunes are disappointingly under-powered. The toxic-edged "Blackout" made her, finally, interesting and wisely she favours material from that album, blasting her way through "Freakshow", "Get Naked (I Got a Plan)", "Radar" and "Oh Oh Baby".

Like Elvis, she's much more enjoyable when she focuses on sex, churning out "Toxic", her memorable 1998 debut "Hit Me Baby One More Time" and her Prince-influenced "I'm a Slave 4 U" with camp aplomb. The scratchy vocal distortions and the bleeps, squeaks and electronic mash-ups on her recordings are also perfectly replicated here. No surprise, given that her "band" isn't visible.

Twenty minutes in and you're longing for a restful acoustic number. However, Brit, who's sheer ordinariness is appealing, is no Joan Baez. She's a construct, the ultimate 21st-century product and, as it turns out, a (mime) survivor who will keep on taking the hits.