The attacks on official websites were only briefly effective but Universal Music remains offline.
Security experts warned that Anonymous was using a new tactic that meant people might unknowingly participate in its attacks.
"In the past, Anonymous has encouraged supporters to install a program called LOIC, which allows computers to join in an attack on a particular website, blasting it with unwanted traffic," said Graham Cluley of Sophos.
"This change in tactic from Anonymous, which allows attacks to be launched by simply clicking on a link, means that internet users need to be extremely careful when clicking on unknown URLs or they could unwittingly be joining this latest zombie army."
It marks a further escalation of the battle between copyright holders, who say the film and music industries are being badly damaged by digital piracy, and those who oppose regulation of the internet.
This week saw an unprecedented protest against stricter enforcement of copyright online by major web organisations including
Wikipedia, which made its English version inaccessible for 24 hours. The blackout was designed to galvanise opposition to Sopa and Pipa, two pieces of legislation under consideration in Congress that would make it easier to cut off pirate websites.
A federal court in Virginia ordered that 18 web address associated with Megaupload.com be seized. Some 20 search warrants were executed in the United States and eight other countries and about $50 million in assets were also seized.
The website acted as a “cyber locker”, allowing users to upload large files for others to download for free. Dozens of similar services exist, but Megaupload.com was the biggest.
Users could also pay at least $9.99 per month for a premium membership, to receive faster downloads. Megaupload.com boasted of accounting for four percent of all traffic on the internet and receiving 50 million visitors per day.
Megaupload.com even claimed support from popular artists including Will.I.am and Kanye West, who both appeared in a video promoting the website.
The two companies associated with Megaupload.com, as well as seven men who allegedly ran it, now, however, face charges of engaging in a racketeering conspiracy, conspiring to commit copyright infringement, conspiring to commit money laundering and criminal copyright infringement.
Four of the men, including Kim Dotcom, the alleged leader of the enterprise, were arrested yesterday in Auckland, New Zealand. A flamboyant 37-year-old German, Dotcom is also known as Kim Schmitz and Kim Tim Jim Vestor, and has several
previous convictions for computer crimes. All the men face up to 20 years imprisonment.
The case against them alleges they did not respond to complaints of infringement by copyright holders and “and deliberately misrepresented to copyright holders that they had removed infringing content”.
“When notified by a rights holder that a file contained infringing content... the conspirators would disable only a single link to the file, deliberately and deceptively leaving the infringing content in place to make it seamlessly available to millions of users to access through any one of the many duplicate links,” the Department of Justice alleged.
It said the Metropolitan Police assisted in the investigation. A spokesman for the Met was not immediately able to say what help British authorities gave.