The group "Anonymous" claimed credit for bringing down the websites of the two firms after they suspended payments to WikiLeaks, and for attacking the site of a Swiss bank that closed an account of site founder Julian Assange.
"Hackers Take Down Visa.com in the Name of Wikileaks. Wow. This is getting crazy," said a message from the group on microblogging site Twitter, as the attacks started to take on the appearance of a cyber war.
The fallout from the US State Department cables released by WikiLeaks, which said it had nothing to do with the hacking, meanwhile continued yesterday with news organisations in the United States and Europe releasing more revelations.
Britain's Guardian newspaper published cables that showed a top Shell executive boasting the oil giant had inserted staff into all levels of the Nigerian government and knew "everything that was being done in those ministries."
Cables posted by the New York Times, meanwhile, recounted how Washington pressured Germany to not issue arrest warrants against CIA officers involved in the 2003 kidnapping of a German citizen mistakenly thought to be involved with Islamic militants.
As Assange spent his first full day in a London prison after he was refused bail on Tuesday, it emerged that one of Britain's highest-profile lawyers will fight moves to extradite him to Sweden to face rape accusations.
WikiLeaks has enraged governments around the world by releasing a wave of US diplomatic cables, detailing everything from China's view of North Korea to unflattering descriptions of world leaders.
After WikiLeaks appealed for donations to be able to continue its activities, Mastercard and Visa said they were suspending payments to the site, sparking attacks on their websites.
The Swiss Post Office banking service, PostFinance, also became a target yesterday after earlier in the week revealing that it had closed an account set up by Assange, saying he had given false information.
Cyber attacks by "Anonymous" knocked all three sites offline yesterday.
Even US conservative icon Sarah Palin was a target of the group following her call to pursue Assange "with the same urgency we pursue Al-Qaeda and Taliban leaders."