LONDON – 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. 

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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. 

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.