The group "Anonymous" has claimed credit for bringing down the websites of Mastercard and Visa after they suspended payments to WikiLeaks, and for attacking the site of a Swiss bank that closed an account of site founder Julian Assange.

"The campaign is not over. It's still going strong, more and more people are joining and helping out. Just last night, Visa went down for a brief period," an unofficial spokesman for the group, calling himself "Coldblood", told BBC Radio.

"I see this as a war, but not your conventional war. This is a war of data. We are trying to keep the Internet open and free for everyone, just the way the Internet has been and always was."

Coldblood, who speaks with a British accent and says he is a software engineer, said Anonymous was "targeting mainly companies which have decided for whatever reason not to deal with WikiLeaks". "Some of the main targets involve Amazon, Mastercard, Visa and PayPal."

He said more people were downloading a so-called "voluntary botnet tool" that allows them to overload the servers of targeted organisation. The same tactic has been employed by unknown opponents of WikiLeaks itself in a bid to drive the website offline.

WikiLeaks supporters reportedly also shut down the website of the Swedish head prosecutor overnight, though there was no immediate claim of responsibility from Anonymous.

Assange, a 39-year-old Australian, is being held pending a hearing on extradition to Sweden where he is wanted for questioning over rape and sexual assault allegations.