Credit card giants MasterCard and Visa came under intense cyber attack on Wednesday as supporters of WikiLeaks retaliated 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, also said it had reported an online attack to police.

Assange's online supporters hit 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 http://www.mastercard.com/ 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.

The same group claimed responsibility for bringing down Visa Inc's site, which was temporarily unavailable in the United States, but later restored.

Visa spokesman Paul Cohen said its processing network is functioning normally and cardholders can continue to use their cards as they routinely would. Account data is not at risk.

Mark Stephens, Assange's principal lawyer in London, denied the WikiLeaks founder had ordered the cyber strikes, which appeared to target companies seen as cooperating with efforts to rein in WikiLeaks.

Assange did not give instructions to hack the company websites, Stephens told Reuters.

MasterCard, calling the attack a concentrated effort to flood our corporate website with traffic and slow access, said all its services had been restored and that account data was not at risk.

But it said the attack, mounted by hackers using simple tools posted on the Web, had extended beyond its website to payment processing technology, leaving some customers unable to make online payments using MasterCard software.

CARRY ON

Assange, who is hailed by supporters as a defender of free speech, is now battling to clear his name. He spent the night in a British jail and will appear for a hearing next Tuesday.

He has lived periodically in Sweden and has been accused of sexual misconduct by two female Swedish WikiLeaks volunteers. The pair's lawyer said their claims were not a politically motivated plot against Assange.

It has nothing to do with WikiLeaks or the CIA, said lawyer Claes Borgstrom, whose website also came under cyber attack, according to officials.

Assange has angered U.S. authorities and triggered headlines worldwide by publishing the secret cables.

Australian Foreign Minister Kevin Rudd said the people who originally leaked the documents, not Assange, were legally liable and the leaks raised questions over the adequacy of U.S. security.

The State Department acknowledged the U.S. government did bear responsibility for the leaks, but said the WikiLeaks releases put real lives and real interests at risk.

WikiLeaks vowed it would continue making public the confidential U.S. cables, only a fraction of which have been published so far.

While most denial of service attacks involve botnets, programs that hijack computers and use them to target individual websites, the latest cyber attacks appear different because the attackers seemed to be using their own computers.

In Washington, U.S. government sources said there was concern that Assange's next batch of material could center on suspected militants held at Guantanamo Bay. They may include

threat assessments by U.S. intelligence agencies gauging the likelihood that specific detainees would return to militant activities if set free.

Those could cause further embarrassment for the Obama administration if they show that detainees deemed likely to return to terrorism were nevertheless released and subsequently involved in anti-U.S. violence.

The original source of the leaked cables is not known, but a U.S. army private, Bradley Manning, who worked as an intelligence analyst in Iraq, has been charged with unauthorized downloading of more than 150,000 State Department cables.

For Visa and MasterCard, the world's two largest credit and debit card processors, the attacks raised questions about the vulnerability of core operations -- and consumers' ability to use credit, debit and online payments instead of cash.

Nevertheless, investors in both companies largely reacted with a shrug. Shares of both companies closed up over 1 percent, although Visa slipped slightly in after-hours trading.

Online payment service PayPal, which was among companies that suspended WikiLeaks' accounts used to collect donations, said it had acted at the behest of the U.S. government.

On November 27th, the State Department, the U.S. government basically, wrote a letter saying that the WikiLeaks activities were deemed illegal in the United States and as a result our policy group had to make the decision of suspending the account, Osama Bedier, PayPal's vice president of platform and emerging technology, told a conference in Paris.

ABC News reported that the website and personal credit card information of Sarah Palin, former Alaska governor and 2008 Republican vice presidential candidate, were also attacked, disrupting her accounts.

Palin, a darling of the conservative U.S. Tea Party movement, has criticized Assange in the past, calling him anti-American.

This is what happens when you exercise the First Amendment and speak against his sick, un-American espionage efforts, she said in an e-mail to ABC.

(Additional reporting by Michel Rose and Peter Apps in London, Emma Thomasson in Zurich, and Patrick Lannin in Stockholm; Mark Hosenball in Washington and Maria Aspan in New York; Writing by Andrew Quinn; Editing by Eric Beech and Christopher Wilson)

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