Hackers supporting the WikiLeaks organization attacked the websites of MasterCard and Visa on Wednesday, retaliating for the gradual shutdown of most channels for the leaker group to receive donations, dpa reported.
Anonymous activists also warned in online statements that their future targets would include online payment service PayPal and Twitter, while the sites of US Senator Joe Lieberman and conservative politician Sarah Palin were also compromised in apparent retaliation for their outspoken criticism of the controversial whistleblower site.
The overwhelming attacks took down the websites of the credit-card companies for several hours Wednesday after they announced they were blocking donations to WikiLeaks through their financial tools.
The attackers claimed over their Twitter account responsibility for the denial-of-service attack and said it was part of "Operation:Payback."
"Operation Payback," their message read. "TARGET: WWW.VISA.COM: : FIRE FIRE FIRE!!! WEAPONS."
Datacell, the Iceland-based commercial arm of WikiLeaks, said it would taken legal action against both credit-card companies.
The personal account of WikiLeaks chief Julian Assange, 39, at a Swiss bank was closed earlier this week.
Assange is in Wandsworth Prison in London after Sweden sought his extradition on forced sex charges. Sweden denies this has any connection with US efforts to bring Assange to justice for gradually releasing 250,000 leaked US diplomatic cables.
Assange has denied the Swedish allegations, saying they are part of a smear campaign.
A German foundation that has been a conduit for donations to WikiLeaks, the Wau Holland Stiftung, said its lawyer was sending a warning to PayPal, charging that about 10,000 euros (13,000 dollars) had been frozen by PayPal, a subsidiary of eBay.
It said this was the amount donated via PayPal from Friday to Saturday, when the PayPal account was finally frozen. It said PayPal's allegation of "illegal activity" was libellous and must be deleted from the PayPal blog.
The only remaining donation channels mentioned Wednesday on the WikiLeaks website were remittances to Wau Holland's bank account in Germany, transfers to a bank account in Iceland or mailing cash or cheques to a post-office box in Melbourne, Australia.
Supporters of Assange were believed to have targeted the website of the Swedish prosecution agency overnight Wednesday, blocking access to the agency's site.
The website was back up Wednesday but users could experience some disturbances, the Prosecution Authority said.
Another target was a website operated by a Swedish lawyer, who represents the two women who filed the complaints against Assange.
Australia pledged consular help to Assange and backed away from its claim that releasing confidential US diplomatic cables had constituted a crime.