A computer server and IT equipment belonging to the alternative media network known as Indymedia have been seized by police in Bristol.
Indymedia says the raid was an attack on press freedom
The raid is understood to have been prompted by complaints about a message on the site concerning rail vandalism.
A 30-year-old man was arrested, and bailed, on suspicion of incitement to commit criminal damage.
A statement on Indymedia UK said: "Police demanded access to the server to gain the IP details of a posting."
A representative of Bristol Indymedia, on behalf of the collective, told BBC News: "Yesterday the police raided a residential property in Bristol and seized an Indymedia server and other computer equipment.
"We see this police action as an attack on the freedom of speech."
Tim Lezard, president of the National Union of Journalists (NUJ), added: "We are obviously not happy that police have closed the server.
"We are supposed to be a free press.
"Will people read a post and take action?"
The raid and arrest were carried out by the British Transport Police.
A spokesman said: "This is not unusual. When we get wind of graffiti, for example, we often do house searches."
Once obtained, the IP address can then be used by internet service providers to track down computer users.
In 2004, servers belonging to Indymedia were seized in London by the FBI, acting on behalf of the Italian and Swiss authorities.
The legal justification for that raid included a gagging order that prevented details being revealed.
However, the servers were thought to have been seized under the Mutual Legal Assistance Treaty which is typically used by nations co-operating to investigate cross-border crimes such as terrorism, kidnapping and money laundering.