The group "Anonymous" has 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, who is being held in the UK.
"The campaign is not over. It's still going strong, more and more people are joining and helping in. Just last night Visa went down for a brief period," an unofficial spokesman for the group, calling himself "Coldblood", told BBC Radio.
"I see this as becoming a war but not your conventional war. This is a war of data, we are trying to keep the Internet open and free for everyone, just the way the Internet has been and always was."
Wikileaks Cyberwar
Coldblood, who speaks with a British accent and says he is a software engineer, said Anonymous was "targeting mainly companies which have decided for whatever reason not to deal with WikiLeaks".
"Some of the main targets involve Amazon, Mastercard, Visa and PayPal."
He said more people were downloading a so-called "voluntary botnet tool" which allows people to overload the servers of targeted groups.
The same tactic has been employed by unknown opponents of WikiLeaks itself in a bid to drive the website offline.
WikiLeaks supporters reportedly also shut down the website of the government of Sweden overnight, though there was no immediate claim of responsibility from Anonymous.
Assange, a 39-year-old Australian, is being held in a London prison pending a hearing on extradition to Sweden, where he is wanted for questioning over rape and sexual assault allegations.
Wikileaks Cyberwar
Comments
We NEED transparency for our global society that we created an cannot control.To many crises. We'd never gone to Iraq if we read the cables first?
Redesign democracy now. It's E-government, not E-commerce tat changes our world (stupid!). How can a few wise leaders alone solve complex global issues pending ? Come on free press, write about the roadmap to E-power-democracy-morevote! If democracy fails, the only solution is More democracy.