The Swedish prosecution authority, whose arrest order for Assange over accusations of sexual offences 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 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.”
“MasterCard is experiencing heavy traffic on its external corporate website – MasterCard.com – but this remains accessible,” MasterCard said in a statement, adding consumer card transactions were not affected.
Assange spent the night in a British jail and will appear for a hearing on December 14.
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.
“Mr Assange is not himself responsible for the unauthorized release of 250,000 documents from the U.S. diplomatic communications network,” Rudd told Reuters in an interview.
“The Americans are responsible for that,” said Rudd, who had been described in one leaked U.S. cable as a “control freak.”
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 campaigning Australian journalist John Pilger and British film maker 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.
The latest cables, reported in Britain’s Guardian newspaper, said Libyan leader Muammar Gaddafi made threats to cut trade with Britain and warned of “enormous repercussions” if the Libyan convicted of the 1988 Lockerbie airline bombing died in a Scottish jail. He was freed in August 2009.
WikiLeaks also released cables on Wednesday that showed Saudi Arabia proposed an “Arab army” be deployed in Lebanon, with U.S. air and naval cover, to stop Shi’ite Hezbollah militia after it seized control of parts of Beirut in 2008.
Like many of the cables, the disclosures give an insight into diplomacy which is normally screened from public view.
The original source of the leaked cables is not known, though 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 now being released by WikiLeaks.
Assange defended his Internet publishing site in a newspaper commentary on Wednesday, saying it was crucial to spreading democracy and likening himself to global media baron Rupert Murdoch in the quest to publish the truth.
(Additional reporting by Michel Rose and Peter Apps in London and Patrick Lannin in Stockholm; editing by Matthew Jones)