LONDON — Britons took to social networking sites on Wednesday to expose the rioters who went on the rampage for four nights, posting photos of masked gangs looting and hurling missiles.

Much of the violence, which started in London but has since spread to other parts of the country, was captured on mobile phone cameras, video recorders or CCTV, and the images quickly found their way into cyberspace.

London's Metropolitan Police made a tentative attempt to use social media to track down suspects, putting up 25 photos of youths breaking into shops and lobbing missiles on photo-sharing site flickr.

But the official effort paled in comparison to the surge of activity by amateur web investigators.

One such project is a web page called "Catch A Looter", which has been set up on blog-hosting website tumblr and features dozens of photos from the London riots.

Images showed looters walking out of shops with electrical goods, clothes and bottles and close-up shots of rioters hurling missiles.

But just a day after it was set up, the creator announced the web page would no longer be updated, saying that with the riots spreading across the country "there are loads of photos and I can barely keep up".

"The beauty of social media is you can put these things up quickly, and take them down again too, without any cost," he said.

Another, more controversial, effort was under way to track down rioters, with a Google Group set up called "London Riots Facial Recognition".

Members of the group are reportedly using technology similar to Facebook's photo recognition tools to identify looters whose faces appear on the web.

It was quickly restricted to current members of people invited to join after concerns were raised in the media following discussions by the group's members on the legality of using facial recognition.

As well as using the web to hunt rioters, 'netizens' also launched a very British "anti-riot" on Wednesday with a Twitter and Facebook campaign entitled "OperationCupOfTea".

People were asked to "Stay in and drink tea" and post a photo of their beverage online, instead of going on the rampage. The project was one of the top trending topics on Twitter.