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The online group Anonymous said Wednesday that it had paralyzed the Egyptian government's Web sites in support of the antigovernment protests.

Anonymous, a loosely defined group of hackers from all over the world, gathered about 500 supporters in online forums and used software tools to bring down the sites of the Ministry of Information and President Hosni Mubarak's National Democratic Party, said Gregg Housh, a member of the group who disavows any illegal activity himself. The sites were unavailable Wednesday afternoon.

The attacks, Mr. Housh said, are part of a wider campaign that Anonymous has mounted in support of the antigovernment protests that have roiled the Arab world. Last month, the group shut down the Web sites of the Tunisian government and stock exchange in support of the uprising that forced the country's dictator, Zine el-Abidine Ben Ali, to flee.

Mr. Housh said that the group had used its technical knowledge to help protesters in Egypt defy a government shutdown of the Internet that began last week. "We want freedom," he said of the group's motivation. "It's as simple as that. We're sick of oppressive governments encroaching on people."

Anonymous also mounted strikes late last year, characterized by some of its supporters as a "cyberwar," against companies like MasterCard, Visa and PayPal that had refused to process donations to the antisecrecy group WikiLeaks.

The F.B.I. said last week that it had executed 40 search warrants "throughout the United States" in connection with that campaign. The strikes by Anonymous, known as "distributed denial of service" attacks, could lead to criminal charges that carry 10-year prison sentences, the F.B.I. said. Arrests have been made and equipment seized in Britain, the Netherlands, Sweden, Germany and France, according to British and American officials. They declined to provide further details.

Barrett Brown, who is helping to organize a legal defense for those who might be prosecuted, said further raids were expected.

Mr. Housh said "these arrests aren't going to have any effect."

Just hours after the raids, he said, about 600 people, including many who had been arrested and then released, were back online and coordinating efforts in Egypt.