Hackers have targeted the US government and copyright organisations following the shutdown of the Megaupload file-sharing website.
The Department of Justice (DoJ), FBI and the Motion Picture Association of America (MPAA) among others have been bombarded with internet traffic.
Web links have been been distributed which, when clicked, make the user's computer part of the attack.
A statement attributed to Anonymous claimed responsibility.
Blackout protest
The DoJ announced on Thursday that it had taken action to force Megaupload and related domain names offline, and had charged the firm’s co-founders and others with violating piracy laws.
Four of the employees have been arrested in Auckland, New Zealand, at the request of the US authorities.
Police also seized cash, valuable cars and a short-barrelled shotgun from the residence of the website’s German founder, Kim Dotcom, formerly known as Kim Schmitz.
They appeared in court on Friday. One of their lawyers initially objected to media requests for photographs, but the accused said that they did not mind “because we have nothing to hide”.
