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.

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.”

Other notable business news:

ICL plant expanding

North Lawrence’s ICL Performance Products plant will undergo an expansion, the company announced Wednesday.

According to a news release, the company will build a new sodium hexametaphosphate plant at the Lawrence facility, 440 N. Ninth St. The new plant — expected to be completed by late 2011 — will help the company meet growing demand in the food additives, beverage and water treatment chemical markets.

An ICL spokeswoman on Wednesday wasn’t able to immediately give details about the size of the project or whether it would add jobs to the Lawrence facility. Instead, such information about the project was expected to be released by ICL’s parent company — which is based in Israel — later this week.