Warte Mal!/Ann-Sofi Siden **

I doubt if all the punters queuing for Paul Klee at the Hayward know that there's a storming show waiting for them upstairs as well: a rambling video installation by a lesser-known Swedish artist, Ann-Sofi Sidèn. Meandering through Warte Mal! (German for Hey wait!) feels like a cross between making your own investigative documentary and taking a crummy European holiday break.

This is Dubi, a town on the Czech-German border that has the dubious privilege of possessing Europe's longest stretch of roadside prostitution.

Not that you'd know this when you first walk into the work. It's August, the nights are long, the bar serves all night and the young clientele are drunkenly dancing, playfighting and wobbling into the street. It only becomes clear in the next room, when each of the characters is interviewed individually in the harsh light of day, that these fragile-looking kids (who are in their late teens and early twenties) are hookers.

Any illicit thrill is instantly sapped as you watch these compelling and candid interviews. Abduction, beatings, drugs and soul-destroyingly low-paid work are recurrent themes. It's a blow in the guts for feminism - but Sidèn's exposè is far from being a monolithic political tract. Everyone is given an equal voice: not just the girls, but their pimp, the owners of the bar, Dubi's head of police, and Sidèn herself (in the form of a slide-projected diary). Everyone is implicated in the corruption, and there are no easily identifiable culprits.

The knotty complexity of Sidèn's installation makes most journalism look even more dumbed-down and biased than usual. Of course, this kind of thing could never be shown on TV: there are some 13 videos to get through, some of which are half an hour long. It's also important that you can flit around from video to video, each one shown in a glass-windowed confessional booth which has overtones of the witness box as well as the peep show.

Warte Mal! has the compulsive attraction of a car crash, and before you know it an hour and half has slipped by - but it's time well spent, and you're haunted by the strength and frailty of the characters long after you leave the gallery. A blinder.

Prostitution after the Velvet Revolution Hayward Gallery, SE1. Until 1 April 2002. Tel: 020 7960 5226 .