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Just hours after MasterCard's website was disabled by WikiLeaks supporters, Visa.com is now down as well.

Via its Twitter account (@Anon_Operation), Anonymous, an activist hacker group, claimed responsibility for the denial of service attack--part of "Operation Payback"--that brought down Visa.com.

"TARGET: WWW.VISA.COM :: FIRE FIRE FIRE!!! WEAPONS http://bit.ly/e6iR3X ::: SET YOUR LOIC TO irc.anonops.net ::: #DDOS #PAYBACK #WIKILEAKS," Anonymous tweeted. Shortly after it posted a tweet that read, " IT'S DOWN! KEEP FIRING!!! #DDOS #PAYBACK #WIKILEAKS."

Anonymous explains that Operation Payback is "an ongoing campaign by Anonymous against major anti-piracy & anti-freedom entities."

MasterCard and Visa are among many sites that have been targeted--and taken down--by "hacktivists." Websites belonging to Swiss bank PostFinance, Senator Joe Lieberman, PayPal, and Sarah Palin have also been disabled.

Like MasterCard, Visa also announced that it would suspend payments to WikiLeaks, a move that has rankled WikiLeaks supporters.

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