Personal data is posted on an activism website. A man is arrested under the Serious Crime Act - but his involvement was minimal

At about 7am on a Monday in February, police arrived at the Sheffield home of a man wanted in connection with the country's largest animal rights extremism case. He was arrested, his home searched and computer equipment seized.

The man, an IT worker who can't be named, is now on police bail on suspicion of offences under the Serious Crime Act 2007. But his case has prompted fears that authorities could use recently adopted and wide-ranging powers to prosecute people for the actions of others online.

He is not accused of threatening or harassing anyone, let alone animal rights terrorism. Instead, the man, in his 40s with a young family, is being investigated for minor involvement with a website that briefly allowed activists to publish the personal information of a senior high court judge in its forums.

The judge in question was Mr Justice Neil Butterfield. In January he handed down lengthy prison sentences to seven members of Stop Huntingdon Animal Cruelty (Shac) after an investigation over several years by Kent police. The next day, an animal rights activist posted his personal information to the comments section of an article on Indymedia UK, a citizen journalism site frequented by protesters.

"You might want to let this friend of [Huntingdon Life Sciences] know exactly what you think about him," the commenter urged other Indymedia UK users. Incitement to harassment is an offence that has in the past landed animal rights activists in prison.

Administrators soon deleted the information in line with the site's privacy policy. A second anonymous commenter reposted it that day, and it was again deleted.

Kent police contacted Indymedia UK the following day, asking for the IP addresses of the commenters, which would enable investigators to identify them via their ISPs' customer records.

The vast majority of websites record IP logs. They provide commercial operations with valuable geographic and demographic data, and are often useful for security professionals battling denial of service or other attacks.

However, Indymedia UK, run on a shoestring by volunteers and with a strong interest in the anonymity of the activists who use it, does not store IP logs. Connecting to one of its servers means your IP address is briefly written to a RAM chip, but administrators have configured the Apache server software not to keep any long-term record on hard disk.

Kent police then seized an Indymedia UK server from UK Grid, a firm in Manchester providing "co-location" services - a spot in a data centre and a connection to the internet. The Sheffield man's name was on the contract.

Kent police contacted him repeatedly by phone, demanding he disclose IP logs. After two weeks of being told the logs did not exist, officers travelled to Sheffield and launched their raid on 9 February.

The man was arrested under provisions of the Serious Crime Act that outlaw "intentionally encouraging or assisting an offence", "encouraging or assisting an offence believing it will be committed" and "encouraging or assisting offences believing one or more will be committed". Officers from Kent questioned the man for several hours with his lawyer present, before releasing him on bail to appear at a police station in May.

Friends of the man said the Indymedia UK server was one of several unrelated machines located at UK Grid under his name. He had been running them as a "hobby", helping NGOs and groups get online. He did not have administrator privileges for the site, they said, and has not been an animal rights activist. "He's pretty stressed out by it all," one said.

An Indymedia UK spokesman agreed the man had nothing to do with the website beyond the co-location contract. "We see the arrest as an attack on the website and on press freedom," he said.

Anthony Burton, a senior partner at media and criminal law firm Simons Muirhead & Burton, said Kent police were breaking new ground using the act in this way. The relevant provisions only came into force on 1 October last year.

"It's a new offence and I'm not aware of any case involving its use," he said. "We're on virgin territory. It will be for the Crown Prosecution Service to decide whether there's a case to answer.

"A defence that may be available is that the man acted reasonably, but that would be for him to prove," Burton said. The police were normally very cautious in their approach to cases involving freedom of speech issues, he said, but the involvement of a senior member of the judiciary had probably prompted them to act.

At the weekend, Kent police said the investigation of the man was ongoing.

Indymedia UK is no stranger to the attentions of law enforcement agencies. In 2004 it was knocked offline by a raid in London on behalf of the FBI, which was investigating violent protests at meetings of the G8. That downtime prompted activists to redesign the site's infrastructure.

The seizure in Manchester had a negligible effect on Indymedia's function as the server was just one of many "mirrors" internationally, all serving the same pages. Similar arrangements are used by the whistleblowing site Wikileaks and the P2P search engine The Pirate Bay, which both fear takedown by authorities.