Credit card giant MasterCard and the Swedish prosecutor’s office came under cyber attack yesterday as WikiLeaks supporters vowed to retaliate for moves against Julian Assange after the release of U.S. diplomatic cables that angered and embarrassed Washington.

The Swedish prosecution authority, whose arrest order for Assange over accusations of sexual offenses led a British court to remand the 39-year-old WikiLeaks website founder in custody, said it had reported the online attack to police.

“Of course, it’s easy to think it has a connection with WikiLeaks, but we can’t confirm that,” prosecution authority Web editor Fredrik Berg told Reuters Television.

Assange supporters also went for the corporate website of credit card firm MasterCard in apparent retaliation for its blocking of donations to the WikiLeaks website.

“We are glad to tell you that is down and it’s confirmed!” said an entry on the Twitter feed of a group calling itself AnonOps, which says it fights against censorship and “copywrong.”

Mark Stephens, Assange’s principal lawyer in London, denied that the WikiLeaks founder had ordered the cyber strikes. Assange “did not give instructions to hack” the company websites, Stephens said.

MasterCard said its systems had not been compromised by what it called “a concentrated effort to flood our corporate website with traffic and slow access.”

Web hack group is grassroots

Advocates of WikiLeaks founder Julian Assange quickly built a cyber army that took down MasterCard’s website using simple tools posted on the Web.

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 attack seems to have come from a grass-roots effort organized by AnonOps, which distributes sophisticated hacking software that is relatively simple for supporters to install on their computers.

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.

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