The websites for Visa and MasterCard were inaccessible for parts of Wednesday, likely the result of attacks by WikiLeaks supporters who are angry that the credit card companies had stopped processing donations to the organization.
Both MasterCard and Visa said that cardholders' accounts were not at risk and that people could continue using their credit cards throughout the day.
Supporters of the WikiLeaks, which has released thousands of classified government documents in recent weeks, said they would attack companies and groups hostile to site and its founder. An Internet group operating under the label "Operation Payback" claimed responsibility for the MasterCard and Visa problems in messages on Twitter and elsewhere.
MasterCard's troubles began in early morning Eastern time and by mid-afternoon, its website was once again operational. But the hacker group appeared to be preparing for its next target, Visa Inc., and by about 4 p.m. EDT the company's corporate website was inaccessible. Spokesman Ted Carr said Visa's processing network, which handles cardholder transactions, was working normally.
The hacking group Anonymous, known for previous attacks on the Church of Scientology and Kiss bassist Gene Simmons, is distributing software tools to allow anyone with a computer and an Internet connection to join in the attacks as part of "Operation Payback." Such tools are widely available on the Internet and can easily launch a large number of attacks on targeted websites, said Dean Turner from the computer security firm Symantec.
MasterCard acknowledged "a service disruption" involving its Secure Code system for verifying online payments, but spokesman James Issokson said consumers could still use their credit cards for secure transactions. Consumers can use credit card companies' websites to find information about the cards, but applying for one and accessing existing accounts are done through the banks that issue the cards.
The credit card companies' troubles took place the same day of attacks on websites for Swedish prosecutors, the Swedish lawyer whose clients have accused founder Julian Assange of sexual crimes and the Swiss authority that froze Assange's bank account.
Besides Visa and MasterCard, a string of U.S.-based companies -- including Amazon.com Inc. and eBay Inc.'s PayPal -- cut ties to WikiLeaks amid intense U.S. government pressure. In a statement, PayPal confirmed that its website has been attacked, which at times slowed the site down but did not "significantly" affect payments.
In a statement, MasterCard said its systems have not been compromised. Earlier Wednesday, the company said the problems appeared to be "the result of a concentrated effort to flood our corporate web site with traffic and slow access."
Such an attack, known as a denial of service, is analogous to thousands of people all calling the same phone number at once, resulting in busy signals for the few who are trying to legitimately get through.
The term "hacktivist" is now widely used to describe politically motivated hackers such as the WikiLeaks supporters, said David Perry, global director of education at security company Trend Micro. Earlier this year, an Ohio college student was sentenced to 30 months in prison for hacking into the websites of conservative pundit Ann Coulter and of former New York mayor Rudy Giuliani.
Operation Payback itself appeared to run into problems Wednesday as many of its sites went down. It was unclear who was behind the counterattack. And Facebook banned the group's page from its site, saying in an e-mailed statement it takes action on content that "promotes unlawful activity." Spokesman Andrew Noyes would not comment on whether Facebook banned any individual users.
On Twitter, the group's site continued to function as of Wednesday evening and representatives for Twitter did not respond to e-mails asking whether this will continue to be the case.
A British judge sent Assange to jail on Tuesday, denying bail after he vowed to fight efforts to be extradited to Sweden in a sex-crimes investigation.
AP Business Writer Michelle Chapman and Technology Writer Peter Svensson in New York contributed to this story.