WikiLeaks supporters have launched cyber attacks against organizations they view as the controversial whistleblower website's enemies, The Daily Telegraph reported.

The hackers forced the organizations' websites to grind to a halt in what they called "Operation Payback." The online activists targeted companies including MasterCard and Visa.

"The group, known as Anonymous and thought to be 1,500 to 2,000-strong, flooded the websites of the credit card companies, and that of the Swedish prosecution authority, with millions of bogus visits," the British newspaper wrote late on Wednesday.

The cyber attacks came after the firms, along with online payment company PayPal, announced they will no longer process donations to WikiLeaks.

The whistleblowing site has recently published part of over 250,000 secret U.S. embassy cables. WikiLeaks came under increasing international pressure after the publication, but world leaders have tried to downplay the adverse impact of the leak.

Former Alaska governor Sarah Palin who recently wondered why WikiLeaks founder Julian Assange was not pursued like al-Qaeda, said her website had also been attacked by the Anonymous group.

"This is what happens when you exercise the First Amendment and speak against his sick, un-American espionage efforts," she told ABC News.