The FBI has shut down some 20 sites which were part of an alternative media network known as Indymedia.
A US court order forced the firm hosting the material to hand over two servers in the UK used by the group.
Indymedia says it is a news source for the anti-globalisation movement and other social justice issues.
The reasons behind the seizure are unclear but the FBI has reportedly said the action was taken at the request of Italian and Swiss authorities.
The servers affected were run by Rackspace, a US web hosting company with offices in London.
It said it had received a court order from the US authorities last Thursday to hand over the computer equipment at its UK hosting facility.
"Rackspace is acting as a good corporate citizen and is cooperating with international law enforcement authorities," said a statement by the company.
It said it was responding to an order issued under the Mutual Legal Assistance Treaty. Under the agreement, countries assist each other in investigations such as international terrorism, kidnapping and money laundering
The reasons behind the action against the Indymedia websites are unclear.
The group said the servers affected had hosted the sites of more then 20 local collectives and audio streams for several radio stations, as well as several other projects.
"Indymedia had been asked last month by the FBI to remove a story about Swiss undercover police from one of the websites hosted at Rackspace," said the group in a statement.
"It is not known, however, whether Thursday's order is related to that incident since the order was issued to Rackspace and not to Indymedia."
'Intolerable and intrusive'
A FBI spokesperson told the AFP news agency that it was not an FBI operation, saying the order had been issued at the request of Italian and Swiss authorities.
The seizure has sparked off protests from journalist groups.
"We have witnessed an intolerable and intrusive international police operation against a network specialising in independent journalism," said Aidan White, general secretary of the International Federation of Journalists.
"The way this has been done smacks more of intimidation of legitimate journalistic inquiry than crime-busting."
The UK site of Indymedia is back up and running but several of the other 20 sites affected are still offline.
In the US, the civil liberties group, the Electronic Frontier Foundation (EFF) said it was working with Indymedia over how to react to the seizures.
"The constitution does not permit the government unilaterally to cut off the speech of an independent media outlet, especially without providing a reason or even allowing Indymedia the information necessary to contest the seizure," said EFF Staff Attorney Kurt Opsahl.
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