A group using the Twitter handle @AnonOperation - which was linked to the WikLeaks site itself - tweeted: "Operation #Payback SITREP: www.visa.com is currently DOWN. FIRE FIRE FIRE!!!!"
The post was retweeted many times, adding to the sense that the attacks were co-ordinated, and WikiLeaks Europe declared it was at "cyberwar" on Twitter.
One tweet mocked MasterCard's advertising slogan with the comment: "There are some things WikiLeaks can't do. For everything else, there's Operation Payback."
Operation Payback Continues
Their attack came after the financial giants announced that they would no longer process donations to the anti-secrecy group.
Websites can crash if enough people attempt to use it at once and hackers can facilitate this by infiltrating other people's computers and directing them to the target site, using them as "slaves" without ever seeing - or perhaps even knowing the physical location of - the machine in question.
A spokesman for the hackers Anonymous, who called himself "Coldblood" and said he was 22 and based in London, told the Daily Mail: "Websites that are bowing to government pressure have become targets.
"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.
Operation Payback Continues
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