Announcing its successful hack on a Twitter account, the group declared, "We will fire at anyone that tries to censor WikiLeaks."
Earlier in the day, Swiss bank PostFinance issued a statement announcing that it had frozen 31,000 euro ($41,000 US) in an account set up as a legal defense fund for WikiLeaks founder Julian Assange.
The bank said it had frozen the account because, in opening it, Assange had claimed residency in Geneva.
"Assange cannot provide proof of residence in Switzerland and thus does not meet the criteria for a customer relationship with PostFinance," the bank said.
As of Monday evening, the PostFinance website was unavailable.
With the financial noose tightening around WikiLeaks even as a legal one tightens around its founder's neck, Operation Payback has effectively declared war on the organizations working to hobble WikiLeaks.
"In these modern times, Internet access is fast becoming a basic human right," the group says in a video posted to YouTube. "Just like any other basic human right, we believe it is wrong to infringe upon it."
The video continues: "To move to censor content on the Internet based on your own prejudice is, at best, laughably impossible. The unjust restrictions you impose on us will meet with disaster, and only strengthen our resolve to disobey and rebel against your tyranny."
WikiLeaks has in recent days been under a deluge of cyber-attacks that led to its DNS registration for its .org URL being taken down, but by mid-Monday the site had reappeared on more than 500 different domains.
News sources in Britain reported late Monday that Assange has arranged to meet with British police on Tuesday, and will likely face a court hearing over an international warrant issued by Sweden in connection with accusations of sexual assault. The criminal probe does not allege non-consensual sex, only that Assange had sex with two women without a condom.
Supporters of the secret-spilling organization argue that the controversies surrounding WikiLeaks -- from the unusual criminal probe against Assange, to banks freezing their funds -- are part of a global campaign to shut down a website that has embarrassed world leaders on numerous occasions.
-- With earlier reporting by Stephen C. Webster
The following video was posted to YouTube by Operation Payback.