Hackers attack those seen as opponents of WikiLeaks

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

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 yesterday, Visa’s website was inaccessible.

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

Late yesterday, 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 US-based companies — including Visa, Amazon.com, PayPal Inc., and EveryDNS — to cut ties to WikiLeaks in recent days amid intense US government pressure. PayPal was not having problems yesterday, but the company said it faced “a dedicated denial-of-service attack’’ on Monday.

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

Osama Bedier, PayPal’s vice president, said the company froze WikiLeaks’ account after seeing a letter from the 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 United States, but the US 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 US cables yesterday. 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 US 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 US companies Visa and MasterCard.

WikiLeaks angered the US government earlier this year when it posted a video showing US troops on a helicopter gunning down two Reuters journalists in Iraq. Since then, the organization has leaked some 400,000 classified US war files from Iraq and 76,000 from Afghanistan, which US military officials said could put people’s lives at risk.

In the past few weeks, the group has begun leaking a massive trove of secret US diplomatic cables.