Online activists have launched a string of attacks against those companies which have denied services to Wikileaks.
Coldblood, an unofficial spokesperson of the Anonymous online group, which is co-ordinating "operation payback", explains the motivation behind the campaign.
Mastercard acknowledged there had been "a service disruption" involving its SecureCode system, adding: "Our core processing capabilities have not been compromised and cardholder account data has not been placed at risk.
"While we have seen limited interruption in some web-based services, cardholders can continue to use their cards for secure transactions globally."
Entries on the Twitter page of Operation Payback, the Anonymous campaign, said the Visa site had been taken down.
Visa's website was later restored and spokesman Ted Carr said its processing network, which handles cardholder transactions, was working normally.
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