The main Web site of MasterCard was knocked offline today in a large distributed denial of service (DDoS) attack apparently launched in retaliation for the credit card company's decision this week to cut off services to WikiLeaks.

Similar, much smaller attacks have also been detected against numerous other sites, including those belonging to U.S. Sen. Joseph Lieberman (I-Conn.) and former Alaska Gov. Sarah Palin, according to security researcher Sean-Paul Correll of PandaLabs. Correll has been maintaining a frequently updated blog on the unfolding attacks.

According to Correll, Visa's Web site appears to have been hit with two separate sets of DDoS attacks. The first was launched around 4 a.m. ET and caused intermittent site outages for several hours. No one has claimed responsibility for those attacks. Then at 4 p.m. today the site was hit again with another DDoS attack, this time by Anonymous. The attacks slowed web traffic to the Visa site considerably, Correll said.

Also targeted have been the Web sites of Swiss payment transaction firm PostFinance, as well as PayPal and EveryDNS, all three of whom terminated services to WikiLeaks after the site began publishing thousands of leaked U.S. State Department cables last week.

In addition, DDoS attacks have been launched against the Swedish prosecutors who are pursuing rape charges against WikiLeaks founder Julian Assange. The site of the lawyer representing the two women who were allegedly assaulted by Assange was also hit, Correll said today.

The attacks have resulted in varying degrees of downtime for the targeted Web sites. MasterCard's site, for instance, was largely unavailable for hours after the DDoS attacks against it began shortly after 4 a.m. ET, Correll said. And the attacks against PostFinance have so far resulted in more than 33 hours of downtime for the firm.

In a brief statement, MasterCard this afternoon said it has made "significant progress" in restoring full service to its corporate site. Despite "limited interruptions," cardholders can continue to use their Mastercards securely, the company said. "Our core processing capabilites have not been compromised and cardholder account data has not been placed at risk."

However, the attacks against Lieberman's Senate site and Palin's sarahpac.com appear to have caused much less disruption. According to statistics on Correll's PandaLabs blog, Lieberman's site was down for barely 12 minutes, while Palin's was knocked offline for about 25 minutes.

An anonymous, loosely affiliated group of hackers and activists, calling themselves Anonymous, claimed responsibility for the attacks against MasterCard and the others. The organizers of the group have an ongoing campaign called Operation:Payback that is focused on attacking entertainment industry Web sites over copyright enforcement issues.

Earlier this week, the group announced its support for WikiLeaks and announced Operation:Avenge Assange, which is targeted at "entities involved in censoring [WikiLeaks'] information."