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 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.

The site is popular with Hollywood celebrities and has been endorsed by music stars such as Kanye West. It was also reported Thursday that Swizz Beatz, a music producer married to the singer Alicia Keys, was its chief executive.

Beatz, whose real name is Kasseem Dean, was not named in the indictment.

The announcement of the indictment on Thursday came one day after Wikipedia, Google and other websites staged a protest against congressional legislation intended to crack down on online piracy.

Anonymous, which has launched previous cyber attacks to protest against alleged web censorship, claimed to have taken down a number of sites, though most were up and running again shortly after midnight (0500 GMT Friday.)

Attempts to access the FBI, Justice Department, Universal Music and the Recording Industry Association of America were unsuccessful Thursday evening, but all except Universal Music appeared to have recovered by Friday.

"The Internet is here. Are you ready for The Year of Cyber War? We are. Rise up and join us to fight for your rights," YourAnonNews, an Anonymous-aligned group, said in a tweet Thursday.

The Justice Department said late Thursday that the downing of its website was "being treated as a malicious act" pending a full investigation.

The Justice Department and FBI had earlier said the seven people charged were "responsible for massive worldwide online piracy of numerous types of copyrighted works, through and other related sites."

They generated more than $175 million in criminal proceeds and caused "more than half a billion dollars in harm to copyright owners," the statement said, by offering pirated copies of movies, TV programs and other content.

French President Nicolas Sarkozy welcomed the US shutdown of Megaupload, saying the site's operators were reaping "criminal profits from the illegal distribution of copyrighted works."

The Justice Department said "others known and unknown" had taken part in the alleged copyright infringement and money laundering.

Megaupload Ltd and another company, Vestor Ltd, were indicted by a grand jury in Virginia and charged with racketeering conspiracy, copyright infringement and conspiring to commit money laundering.

Among those indicted was Megaupload founder and sole shareholder of Vestor, Kim Dotcom, 37, a resident of Hong Kong and New Zealand who is also known as Kim Schmitz and Kim Tim Jim Vestor.

The others charged are Finn Batato, 38, of Germany; Julius Bencko, 35, of Slovakia; Sven Echternach, 39, of Germany; Mathias Ortmann, 40, of Germany; Andrus Nomm, 32, of Estonia; and Bram van der Kolk, 29, of the Netherlands.

The Justice Department and FBI said Dotcom, Batato, Ortmann and van der Kolk were arrested on Thursday in Auckland, New Zealand, by local authorities based on arrest warrants requested by the United States.

Bencko, Echternach and Nomm remain at large.

The Justice Department and FBI said $50 million in assets were seized during the operation along with 18 domain names. Attempts to connect to were unsuccessful late Thursday and early Friday.

Conspiracy to commit racketeering and conspiracy to commit money laundering each carry maximum sentences of 20 years in prison while five years in prison is the maximum punishment for the copyright infringement charges.

New Zealand police confirmed four people were arrested in Auckland and would face court on Friday following a US request for their extradition.

The police said New Zealand authorities had been working on the case with the FBI and US Justice Department for several months.