Credit card giants MasterCard and Visa came under intense cyber attack 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.”

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

Sources told Reuters that WikiLeaks’ next release may highlight U.S. government reports on suspected militants held at Guantanamo Bay, which some U.S. officials fear could show some detainees were freed despite intelligence assessments they were still dangerous.

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 web site with traffic and slow access,” said Wednesday all of its services had been restored and that card holder 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.

Assange spent the night in a British jail and will appear for a hearing next Tuesday.

CARRY ON

Assange, who has lived periodically in Sweden, was accused this year 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 U.S. State Department said that while the U.S. government did have responsibility for the leaks, it was important to highlight that the Wikileaks releases “put real lives and real interests at risk.”

WikiLeaks vowed it would continue making public details of the confidential U.S. cables. Only a fraction of them have been published so far.

Assange has become the public face of WikiLeaks, hailed by supporters including Australian journalist John Pilger and British filmmaker Ken Loach as a defender of free speech, but he is now battling to clear his name.

Some supporters appear to want to help him. While most denial of service attacks involve botnets, programs that hijack computers and use them to target individual websites and bring them down, the current cyber attacks seem to be different.

“In this case… they seem to be using their own computers,” he said. Asked what that said about how many individuals might be involved: “Probably hundreds at the least, could be thousands,” said Mikko Hypponen, chief research officer of Finnish software security firm F-Secure.

For Visa and MasterCard, the world’s two largest credit and debit card processors, the attack 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. “You don’t see this being a long-term or terminal situation where they can’t find a fix,” said Michael Nix, principal at Greenwood Capital Associates, which owns Visa shares.

A spokesman for Visa did not immediately return calls seeking comment.

Online payment service PayPal, which was among companies which 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.

Swiss Postfinance, the banking arm of state-owned Swiss Post, which also closed a WikiLeaks donation account, said it had taken countermeasures and an earlier wave of cyber attacks appeared to be waning.

“The community around Julian Assange have said, ‘We’re leaving it now, we’ve shown what we can do. The community has decided to go for Mastercard and Visa now,’ Postfinance spokesman Alex Josty said.

REVELATIONS CONTINUE

As Washington continued to assess the impact of Wikileaks’ release of thousands of classified embassy cables, which include damagingly candid assessments of foreign leaders, some officials were bracing for their next headache.

U.S. government sources said there was concern Assange’s next batch of material could center on suspected militants held at Guantanamo Bay, and 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, although a U.S. army private, Bradley Manning, who worked as an intelligence analyst in Iraq, has been charged by military authorities with unauthorized downloading of more than 150,000 State Department cables.

U.S. officials have declined to say whether those cables are the same ones being released by WikiLeaks.