Supporters of WikiLeaks founder Julian Assange quickly built a cyber army that shut down the websites of the world's two biggest credit card companies using simple tools posted on the Internet.

MasterCard Inc's website was down for much of on Wednesday as a group calling itself AnonOps organized a Denial of Service attack on the credit card giant, which had stopped processing donations for WikiLeaks after the United States criticized its release of sensitive diplomatic cables.

Visa Inc's site was temporarily unavailable late on Wednesday in the United States and the same group claimed responsibility for bringing it down.

Denial of service attacks typically use botnets, or armies of computers that have been enslaved by criminal hackers, to bring down a website by hitting it with an overwhelming number of simultaneous requests for information.

But the MasterCard and Visa attacks seem to have come from a grass-roots effort organized by AnonOps.

It distributes sophisticated hacking software that is relatively simple for supporters to install on their computers.

It is very easy to launch these attacks, said John Bumgarner, chief technology officer for the U.S. Cyber Consequences Unit, which monitors cyber attacks.

The group used Twitter to seek supporters, referring them to its website, where they could download the software that turns a Windows or Mac PC into a weapon against the MasterCard site.

Sean-Paul Correll, a researcher with anti-virus firm PandaLabs who has been monitoring the activities of AnonOps, said the group had thousands of PCs in its volunteer army.

This group of people has grown frustrated with trying to protest, he said. They've finally figured out they can use technology to fight back, that they don't have to stand in a picket line.

WikiLeaks has come under international scrutiny after releasing a slew of diplomatic cables that angered and embarrassed Washington. Assange was arrested in Britain on Tuesday over accusations of sexual offenses.

The attacks on Visa and MasterCard are apparent retaliation for the two largest credit and debit card processing company's move to block donations to the WikiLeaks website.

The AnonOps site links users to a chatroom, where some ask questions about how to use the software and others gloat about their success.

MasterCard still down. Hell yeah, said one anonymous participant in the makeshift chatroom.

Good job everyone! said another user.

MasterCard said the attack did not compromise its core payment processing capabilities, but that there had been some limited interruptions to web-based services it offers customers. It did not elaborate.

Officials with Visa could not immediately be reached for comment.

(Reporting by Jim Finkle; Editing by Xavier Briand and Stacey Joyce)