Meanwhile MasterCard rival Visa, which has also been under a DDoS attack, was finally knocked offline this afternoon. Visa's main corporate site appears to have been hit by two separate attacks according to Sean-Paul Correll, a researcher with PandaLabs. Correll has been maintaining a regularly updated blog on the unfolding attacks.

The first attacks against the site started last night after midnight ET and resulted in intermittent service disruptions for several hours. No group has so far claimed responsibility for those attacks, Correll said.

Then at about 4 p.m. ET today, the company was hit with another DDoS attack -- this time by Anonymous, the group of loosely affiliated hackers that has vowed to attack organizations seen as attempting to censor WikiLeaks.

In a statement, Visa said that its corporate Web site was "currently experiencing heavier than normal traffic" and said it hoped to restore full site operations in the next few hours. "Visa's processing network, which handles cardholder transactions, is functioning normally and cardholders can continue to use their cards as they routinely would. Account data is not at risk."

Anonymous, which has also been attacking entertainment industry sites over copyright enforcement issues, this week launched Operation: Avenge Assange. It is targeted at "entities involved in censoring [WikiLeaks'] information."