PayPal is the latest WikiLeaks opponent to be hit with a denial of service attack by anti-piracy group Operation Payback, joining the growing ranks of WikiLeaks opponents that have been targeted.

Operation Payback has also indicated that Amazon is its next target.

"The attacks slowed the Web site itself down for a short while, but did not significantly impact payments," said a PayPal spokesperson.

Last week, PayPal permanently restricted the account used by WikiLeaks "due to a violation of the PayPal Acceptable Use Policy." In a blog post on Wednesday, PayPal general counsel John Muller said that the company's "difficult decision was based on a belief that the WikiLeaks Web site was encouraging sources to release classified material, which is likely a violation of the law by the source."

Muller said PayPal will release the remaining funds to the foundation that was raising money for WikiLeaks, though the organization's account will remain restricted.

Operation Payback, operated by the clandestine group known as "Anonymous," which reportedly includes members of the "/b/" bulletin board 4chan.org, seems to be targeting the list of companies that have yanked support for WikiLeaks.

On Wednesday, those targets included credit card companies Mastercard and Visa, both of which have ceased processing WikiLeaks donations.

Operation Payback was using the @Anon_Operation Twitter feed to communicate about those attacks, but Twitter suspended that account on Wednesday. Today, the group re-appeared via @Op_Payback.

"target:www.paypal.com status: down. Keep firing your lazors! #ddos #wikileaks #payback #paypal" said a Thursday tweet from @Op_Payback.

The attacks show no signs of stopping. According to its Twitter feed, Operation Payback is trying to cripple Amazon.com next. "Target: www.Amazon.com locked on!!!" said a tweet.

WikiLeaks moved to Amazon Web Services after it was hit with its own denial of service attack, but Amazon yanked support, saying that WikiLeaks violated its terms of use.

"Amazon Web Services does not pre-screen its customers, but it does have terms of service that must be followed," Amazon said in a statement. "WikiLeaks was not following them. There were several parts they were violating."

Amid waning WikiLeaks support from companies in the U.S., Philadelphia startup Xipwire, a financial processing service, has agreed to handle WikiLeaks donations.

"While people may or may not agree with WikiLeaks, we at Xipwire believe that anyone who wishes to support the organization through a donation should be able to do so," said a WikiLeaks editor Julian Assange, meanwhile, will remain in a London jail until at least December 14 after being arrested this week on charges of rape. Assange has denied any wrongdoing.