Hackers attacked credit card giants Mastercard and Visa

Mastercard payments were disrupted but the firm said there was "no impact" on people's ability to use their cards. The attacks came after the Anonymous group of hackers pledged to pursue firms that have withdrawn services from Wikileaks.

Visa's website now appears to be getting slow and experiencing problems. The attacks came after both companies stopped processing payments to the whistle-blowing site.

Entries on the Twitter page of Operation Payback, the Anonymous campaign, said the Visa site had been taken down.

"Hackers Take Down Visa.com in the Name of Wikileaks. Wow. This is getting crazy," read one message on the page. In particular, it said that an authentication service for online payments known as Mastercard's SecureCode, had been disrupted.

It said that it has hit several targets, including the website of the prosecutors who are acting in a legal case against Wikileaks founder Julian Assange. DDoS attacks are illegal in many countries, including the UK.

Coldblood admitted that such attacks "may hurt people trying to get to these sites" but said it was "the only effective way to tell these companies that us, the people, are displeased". Anonymous is also helping to create hundreds of mirror sites for Wikileaks, after its US domain name provider withdrew its services.

Before the Mastercard attack, a member of Anonymous, who calls himself Coldblood, told the BBC that "multiple things" were being done to target companies that had stopped working with Wikileaks or which were perceived to have attacked the site. "As an organisation we have always taken a strong stance on censorship and freedom of expression on the internet and come out against those who seek to destroy it by any means."

"We feel that Wikileaks has become more than just about leaking of documents, it has become a war ground, the people vs. the government," he said. Anonymous, which claimed to have carried out the attack, is a loose-knit group of hacktivists, with links to the notorious message board 4chan.

Some of the early DDoS hits failed to take sites offline, although that was not the point of the attacks, according to Coldblood. DDoS attacks are illegal in many countries, including the United Kingdom.

"The idea is not to wipe them off but to give the companies a wake-up call," he said. "Companies will notice the increase in traffic and an increase in traffic means increase in costs associated with running a website."

Coldblood admitted that such attacks "may hurt people trying to get to these sites" but said it was "the only effective way to tell these companies that us, the people, are displeased".