MasterCard, which pulled the plug on its relationship with WikiLeaks on Tuesday, also seemed to be having severe technological problems.
The online vengeance campaign appeared to be taking the form of denial of service attacks in which computers across the Internet are harnessed - sometimes surreptitiously - to jam target sites with mountains of requests for data, knocking them out of commission.
The online attacks are part of a wave of online support for WikiLeaks that is sweeping the Internet. Twitter was choked with messages of solidarity Wednesday, while the site's Facebook page hit 1 million fans.
Offline, the organization is under pressure on many fronts. Assange, its founder, is in a U.K. prison fighting extradition to Sweden over the sex crimes case, while moves by Swiss Postfinance, MasterCard, PayPal Inc. and others have impaired the secret-spilling group's ability to raise money. The U.S. government is also investigating whether Assange can be prosecuted for espionage or other offenses.
Per Hellqvist, a security specialist with the firm Symantec, said a loose network of web activists called "Anonymous" appeared to be behind the attacks. The group, which has previously focused on the Church of Scientology and the music industry, has promised to come to Assange's aid by knocking offline websites seen as hostile to WikiLeaks.
"While we don't have much of an affiliation with WikiLeaks, we fight for the same reasons," the group said in a statement on its website. "We want transparency and we counter censorship. ... This is why we intend to utilize our resources to raise awareness, attack those against and support those who are helping lead our world to freedom and democracy."
It was not immediately clear which attacks the group was responsible for, although activists on Twitter and other forums cheered the news of each one in turn.
The website for MasterCard, which has said it will no longer process donations to WikiLeaks, was either down or sluggish early Wednesday. The company said it was experiencing "heavy traffic" but did not elaborate.
The website for Swedish lawyer Claes Borgstrom, who represents the two women at the center of Assange's sex crimes case, was unreachable Wednesday.
The Swiss postal system's financial arm, Postfinance, which shut down Assange's new bank account on Monday, was also having trouble. Spokesman Alex Josty said the website buckled under a barrage of traffic Tuesday but the onslaught seems to have eased off.
"Yesterday it was very, very difficult, then things improved overnight," he told The Associated Press. "But it's still not entirely back to normal."
While one Internet company after another has cut its ties to the websites amid intense U.S. government pressure - Amazon.com, PayPal Inc., EveryDNS - the French government's effort to stop a company there from hosting WikiLeaks has failed - at least for now.
The Web services company OVH, which is among those hosting the current site - wikileaks.ch - sought a ruling by two courts about the legality of hosting WikiLeaks in France. The judges said this week they couldn't decide on the highly technical case right away.
WikiLeaks evoked the ire of the U.S. government last spring when it posted a gritty war video taken by Army helicopters showing troops gunning down two unarmed Reuters journalists. Since then, the organization has leaked some 400,000 classified U.S. war files from Iraq and 76,000 from Afghanistan that U.S. military officials say included names of U.S. informants and other information that could put people's lives at risk.
The latest leaks have involved private U.S. diplomatic cables that included frank U.S. assessments of foreign nations and their leaders.
Malin Rising in Stockholm, Frank Jordans in Geneva, Jamey Keaten in Paris, Cassandra Vinograd in London, Rod McGuirk in Canberra, Tia Goldenberg in Jerusalem, and Anne Flaherty in Washington contributed to this report.