In what the federal authorities on Thursday called one of the largest criminal copyright cases ever brought, the Justice Department and the Federal Bureau of Investigation seized the Web site Megaupload and charged seven people connected with it with running an international enterprise based on Internet piracy.

Coming just a day after civil protests in the United States over proposed antipiracy bills, the arrests were greeted almost immediately with digital Molotov cocktails. The hacker collective that calls itself Anonymous attacked the Web sites of the Justice Department and several major entertainment companies and trade groups in retaliation for Mega-upload’s seizure. The Justice Department’s site and several others remained inaccessible for much of Thursday afternoon.

Megaupload, one of the most popular so-called locker services on the Internet, allowed users to anonymously transfer large files like movies and music. Media companies have long accused it of abetting copyright infringement on a vast scale. In a grand jury indictment, Megaupload is accused of causing $500 million in damages to copyright owners and of making $175 million through selling ads and premium subscriptions.

Four of the seven people, including the site’s founder, Kim Dotcom (born Kim Schmitz), have been arrested in New Zealand, the authorities said; the three others remain at large. Each of the seven people — who the indictment said were members of a criminal group it called “Mega Conspiracy” — is charged with five counts of copyright infringement and conspiracy. The charges could result in more than 20 years in prison.