Activists fighting on behalf of WikiLeaks brought down MasterCard's Web site today, according to the BBC and other sources.

The hacking group Anonymous is taking responsibility for a distributed denial of service (DDoS) attack on the credit company's site in retaliation for MasterCard's decision on Monday to block donations and payments to WikiLeaks, which was first reported by CNET.

MasterCard's main site was at times either offline or extremely slow to load this morning. Ping requests to the site also timed out, an indication that the site was unreachable.

Early this morning, the Anonymous group, which uses the Twitter account Anon_Operation, tweeted the following statement:

"WE ARE GLAD TO TELL YOU THAT http://www.mastercard.com/ is DOWN AND IT'S CONFIRMED"

MasterCard claimed its site was still functioning but just experiencing heavy traffic on its external corporate site, according to the BBC. The company added that there was "no impact" on the ability of customers to use their credit cards for transactions, the BBC further reported.

The decision to target MasterCard follows attacks by Anonymous against PayPal and other sites for their actions against WikiLeaks.

On Saturday, PayPal cut off access to WikiLeaks to prevent people from donating money to the site. In response, the Anonymous group used a DDoS attack to hit the PayPal blog that announced the payment cutoff.

PayPal was given a chance to explain its decision at LeWeb conference in Paris today. Osama Bedier, PayPal's vice president of platform, mobile, and new ventures, was asked by a reporter why the company cut off access to WikiLeaks, according to TechCrunch. In response, Bedier discussed PayPal's accessible use policy designed to protect customers but also seemed to put the onus for the decision on the U.S. State Department's designation of WikiLeaks as "illegal."

WikiLeaks has come under fire repeatedly--most recently by the U.S. government for its release of classified U.S. State Department documents. Some in government have even called for the site to be classified as a terrorist group.

Arrested in London yesterday, WikiLeaks leader Julian Assange is currently being held without bail as he waits to be extradited to Sweden to face sexual assault charges. Assange has denied the accusations, which his supporters claim have been trumped up as a way to silence him.