A group of journalists has launched a legal action against Scotland Yard after discovering that the Metropolitan police has been recording their professional activities on a secret database designed to monitor so-called domestic extremists.

The six journalists have obtained official files that reveal how police logged details of their work as they reported on protests. One video journalist discovered that the Met police had more than 130 entries detailing his movements, including what he was wearing, at demonstrations he attended as a member of the press.

They have started the legal action to expose what they say is a persistent pattern of journalists being assaulted, monitored and stopped and searched by police during their work, which often includes documenting police misconduct.

In legal paperwork, the journalists who have worked for national newspapers describe how they have regularly exposed malpractice by the state and big corporations and have campaigned for press freedom.

The group includes a journalist on the Times. Jules Mattsson, who, police noted, was “always looking for a story”. Mattsson said that when he had been a victim of crime, police had transferred on to the domestic extremism database details of his appearance, childhood and a family member’s medical history.

Adrian Arbib, a press photographer for three decades, found that police had recorded him taking photographs of Heathrow airport for a Guardian story about the decline of English orchards.

Freelance photographer Jess Hurd discovered that her file dated back to 2000.

Five of the journalists have successfully sued the police in the past, winning damages or apologies over wrongdoing such as being assaulted or unjustifiably searched by officers while they worked.

The legal action, backed by the National Union of Journalists (NUJ), has been initiated at a time when the media are protesting that police are misusing surveillance powers to access reporters’ telephone records to unmask confidential sources.

The journalists are seeking to force the Met to destroy the files held on them as they say the surveillance violates the liberty of the press and their privacy.

They have used the Data Protection Act to obtain copies of the files. Many of the 130-plus entries in the file of freelance video journalist Jason Parkinson since 2005 note that he is a photographer working for the media and log how he is using a camera or video to record events.

Several entries note that he is a member of the NUJ and had a visible card around his neck declaring this fact. Another entry records that on a 2009 demonstration, he was “present throughout the protest, undertaking his photographic duties”. Often his dress, body piercings, and facial hair are described.

The file on David Hoffman, a freelance photographer who has been chronicling demonstrations for the media since 1976, appears to label him twice as “an Anti-Nazi League photographer”. He said he had never been a member or worked for the Anti-Nazi League, a campaign that was wound up more than a decade ago.

He questioned why there was a file on him on the database compiled by the Met’s domestic extremism unit. “I have never contemplated any sort of extreme action of a political or criminal nature”, he said.

The covert unit has been tracking thousands of campaigners and storing files on them. It says it is seeking to pinpoint the minority who have, or are about to, commit crime to promote their political aims.

The Met says the domestic extremism unit deleted many files recently after a watchdog found there was no justification for retaining them.

There has been criticism that many on the database, including Jenny Jones, the Green party’s sole peer, have no criminal record. One campaigner discovered that police had recorded how he had sold anti-capitalist publications from a stall at the Glastonbury festival.

Police are going to the supreme court in December to overturn a landmark ruling that they unlawfully kept the political activities of 89-year-old peace campaigner, John Catt, on the database.

A freedom of information request last month found that the unit has more than 2,000 references in its files to journalists and press photographers.

Bernard Hogan-Howe, the Commissioner of the Metropolitan Police, told MPs

last week that this did not mean that there were that number of files on

individuals, as terms such as “journalists” were used to describe events,

indicating for instance that there were a group of them at a demonstration.

The commissioner said police did not “routinely collect” information on journalists and their sources, adding that the police only took an interest in people if they were either victims or suspects.

The file on journalist and comedian Mark Thomas, who is part of the legal action, contained more than 60 pieces of intelligence in what he has described as “being wonderfully odd in an Ealing Comedy-meets-the-Stasi sort of way”.

Michelle Stanistreet, the NUJ general secretary, said: “There is no justification for treating journalists as criminals or enemies of the state”.

The Met confirmed it had received a legal letter from Bhatt Murphy, the law firm working for the journalists, and would respond in due course.

• This article was amended on 21 November 2014. Jason Parkinson, on whom the Met police have more than 130 entries, is a freelance video journalist, not a photographer. This has been corrected.

