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.
More here.