US authorities have shut down one of the largest file-sharing websites and charged seven people with copyright crimes, sparking a retaliatory cyber attack on the FBI and Justice Department websites.

The two government sites were up and running again early Friday after being shut down for several hours in an attack claimed by the "Anonymous" hacktivist group, which also briefly disabled music and recording industry websites.

The file-sharing site Megaupload.com went offline as officials and the Federal Bureau of Investigation laid out the details of what they described as "among the largest criminal copyright cases ever brought by the United States."

The founder of the Hong Kong-based Megaupload site was among four people arrested for online piracy and crimes that justice officials said had illegally netted them millions of dollars from subscriptions and advertising revenue.