LONDON (AP) — Hackers rushed to the defense of WikiLeaks on Wednesday, launching attacks on MasterCard, Visa, Swedish prosecutors, a Swiss bank and others who have acted against the site and its jailed founder Julian Assange.

Internet “hacktivists” operating under the label “Operation Payback” claimed responsibility in a Twitter message for causing severe technological problems at the website for MasterCard, which pulled the plug on its relationship with WikiLeaks a day ago.

MasterCard acknowledged “a service disruption” involving its Secure Code system for verifying online payments, but spokesman James Issokson said consumers could still use their credit cards for secure transactions. Later Wednesday, Visa’s website was inaccessible.

The online attacks are part of a wave of support for WikiLeaks that is sweeping the Internet. Twitter was choked with messages of solidarity for the group, while the site’s Facebook page hit 1 million fans.

Late Wednesday, Operation Payback itself appeared to run into problems, as many of its sites went down. It was unclear who was behind the counterattack.

MasterCard is the latest in a string of U.S.-based Internet companies — including Visa, Amazon.com, PayPal Inc. and EveryDNS — to cut ties to WikiLeaks in recent days amid intense U.S. government pressure. PayPal was not having problems Wednesday but the company said it faced “a dedicated denial-of-service attack” on Monday.

WikiLeaks’ extensive releases of secret U.S. diplomatic cables have embarrassed U.S. allies, angered rivals, and reopened old wounds across the world. U.S. officials in Washington say other countries have curtailed their dealings with the U.S. government because of WikiLeaks’ actions.

PayPal Vice President Osama Bedier said the company froze WikiLeaks’ account after seeing a letter from the U.S. State Department to WikiLeaks saying that the group’s activities “were deemed illegal in the United States.”

Offline, WikiLeaks was under pressure on many fronts. Assange is in a British prison fighting extradition to Sweden over a sex crimes case. Recent moves by Swiss Postfinance, MasterCard, PayPal and others that cut the flow of donations to the group have impaired its ability to raise money.

Neither WikiLeaks nor Assange has been charged with any offense in the U.S., but the U.S. government is investigating whether Assange can be prosecuted for espionage or other offenses. Assange has not been charged with any offenses in Sweden either, but authorities there want to question him about the allegations of sex crimes.

Undeterred, WikiLeaks released more confidential U.S. cables Wednesday. The latest batch showed the British government feared a furious Libyan reaction if the convicted Lockerbie bomber wasn’t set free and expressed relief when they learned he would be released in 2009 on compassionate grounds.

Another U.S. memo described German leader Angela Merkel as the “Teflon” chancellor, but she brushed it off as mere chatter at a party. American officials were also shown to be lobbying the Russian government to amend a financial bill they felt would disadvantage U.S. companies Visa and MasterCard.

The most surprising cable of the day came from a U.S. diplomat in Saudi Arabia after a night on the town.

“The underground nightlife of Jiddah’s elite youth is thriving and throbbing,” the memo said. “The full range of worldly temptations and vices are available — alcohol, drugs, sex — but all behind closed doors.”

The pro-WikiLeaks vengeance campaign on Wednesday appeared to be taking the form of denial-of-service attacks in which computers are harnessed — sometimes surreptitiously — to jam target sites with mountains of requests for data, knocking them out of commission.

Per Hellqvist, a security specialist with the firm Symantec, said a network of web activists called Anonymous — to which Operation Payback is affiliated — appeared to be behind many of the attacks. The group, which has previously focused on the Church of Scientology and the music industry, is 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. “We want transparency and we counter censorship … 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.”

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 bank account on Monday, was also having trouble. Spokesman Alex Josty said the website buckled under a barrage of traffic Tuesday.

“Yesterday it was very, very difficult, then things improved overnight,” he told the AP. “But it’s still not entirely back to normal.”

Ironically, the microblogging site Twitter — home of much WikiLeaks support — could become the next target. Operation Payback posted a statement claiming “Twitter you’re next for censoring Wikileaks discussion.”

Some WikiLeaks supporters accuse Twitter of preventing the term “WikiLeaks” from appearing as one of its popular “trending topics.” Twitter denies censorship, saying the topics are determined by an algorithm.

Twitter’s top trending topics are not the ones people are discussing the most overall, but those they are talking about more right now than they did previously, Twitter explained in an e-mail Wednesday. If tweets were ranked by volume alone, the weather or other mundane topics would dominate the trends.

WikiLeaks angered the U.S. government earlier this year when it posted a video showing U.S. troops on a helicopter gunning down two Reuters journalists in Iraq. Since then, the organization has leaked some 400,000 classified U.S. war files from Iraq and 76,000 from Afghanistan, which U.S. military officials say could put people’s lives at risk. In the last few weeks, the group has begun leaking a massive trove of secret U.S. diplomatic cables.

U.S. officials have directed their anger at Assange, but others have begun to ask whether Washington shares the blame for the diplomatic uproar.

“The core of all this lies with the failure of the government of the United States to properly protect its own diplomatic communications,” Australian Foreign Minister Kevin Rudd said Wednesday, criticizing the fact that tens of thousands of U.S. government employees had access to the cables.

Assange, meanwhile, faces a new extradition hearing in London next week where his lawyers plan to reapply for bail. The 39-year-old Australian denies two women’s allegations in Sweden of rape, molestation and unlawful coercion, and is fighting his extradition to Sweden.

In a Twitter message Wednesday, WikiLeaks spokesman Kristinn Hrafnsson shrugged off the challenges.

“We will not be gagged, either by judicial action or corporate censorship … WikiLeaks is still online,” Hrafnsson said.

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Malin Rising in Stockholm, Frank Jordans in Geneva, Jamey Keaten in Paris, Cassandra Vinograd in London, Rod McGuirk in Canberra, Brian Murphy in Dubai, Tia Goldenberg in Jerusalem, Michelle Chapman, Peter Svensson and Barbara Ortutay in New York and Anne Flaherty in Washington contributed to this report.