MasterCard's Web site was still unavailable as of mid-morning after having been offline for several hours. Several hacker groups that have expressed support for WikiLeaks claimed credit for the attacks, including Anonymous and a group that calls itself Operation: Payback.
WikiLeaks allies said they attacked MasterCard in retribution for the company's decision to block WikiLeaks from its payments network, a move that made it more difficult for the rogue Web site to collect donations from supporters.
"The reason is amazingly simple," said Anonymous member Gregg Housh. "We all believe information should be free, and the Internet should be free," said Housh, in an interview with The New York Times published Monday.
WikiLeaks has also been cut off by PayPal, and Amazon booted it from its Web hosting servers for violating its Terms of Service agreement.
Meanwhile, Assange is in jail in London fighting an extradition request by Swedish authorities who have charged him with rape and other sex crimes. Assange has denied the charges, which relate to complaints filed by two Swedish women.
U.S. authorities are also said to be mulling espionage and other charges against Assange who, through WikiLeaks, provided several major newspapers—including the Times, The Guardian, Le Monde, and Der Spiegel—with excerpts from classified diplomatic documents.
The leaked documents revealed serious concerns within the U.S. diplomatic community about the resolve and trustworthiness of several key allies, including Afghanistan and Pakistan, in the war on terror.
They also disclosed Saudi Arabia's wish for a U.S. military strike against Iran, and painted unflattering pictures of Western leaders like German Chancellor Angela Merkel and Italian Prime Minister Silvio Berlusconi.
The documents also raise questions about whether British authorities released a prisoner jailed in connection with the Pan Am 103 bombing over Lockerbie to preserve oil deals with Libya.