WASHINGTON (AFP) - Organizers of "Anonymous," the group behind cyber attacks on Mastercard.com and other websites, vowed Wednesday to extend their campaign to anyone with an "anti-WikiLeaks agenda."

In an online chat with Agence France-Presse, organizers of "Anonymous" said thousands of volunteers were taking part in their defense of WikiLeaks and its founder Julian Assange, whom they described as a "free-speech martyr."

"We started off with a small amount of users (below 50)," they said. "Now, we are at around 4,000."

"Anonymous" launched their first distributed denial of service, or DDoS, attack on Saturday, taking down PayPal's official blog, ThePayPalBlog.com, for at least eight hours.

Since then they have targeted the websites of other companies which have severed ties to Assange or WikiLeaks, including Mastercard and the Swiss Post Office bank.

The website of the Swedish prosecutor's office pursuing Assange on allegations of sex crimes was also hit as was the website of the lawyer representing two Swedish women in the case.

"The current target is Mastercard.com, but anyone that has an anti-Wikileaks agenda is within our scope of attack," they said in the online chat with AFP.

"We recruit through the Internet, that means, everywhere: imageboards, forums, Facebook, Twitter... you name it, we're using it," they said, adding that members were drawn from "all over the world."

The Anonymous organizers said the defense of WikiLeaks was an extension of "Operation Payback," which began in September and involved cyber attacks on the websites of the Motion Picture Association of America and Recording Industry Association of America over their copyright protection efforts.

"Operation Payback stands for free speech and no censorship, which is also what Assange is going for," they said. "The general motivation is freedom of information/speech, which is currently under attack.

"When we think we made our point (e.g. WikiLeaks accepted as whistleblower, without fear that they will be prosecuted), we will return to fighting 'copywrong,'" they said.

In a classic DDoS attack, a "botnet" of zombie computers, machines infected with viruses, are commanded to simultaneously visit a website, overwhelming its servers, slowing service or knocking it offline completely.