John Catt talking about the surveillance in 2010 guardian.co.uk

An 87-year-old man is launching a landmark lawsuit against police chiefs who labelled him a "domestic extremist" and secretly recorded his political activities in minute detail.

Lawyers for John Catt are due to open the legal action at the high court on Thursday against a clandestine police unit that has been at the centre of controversy over its undercover infiltration of political groups.

Catt, who has no criminal record, was "shocked and terrified" when he discovered that police had kept a detailed record of his presence at more than 55 demonstrations over a four-year period.

The police had detailed how the Brighton pensioner took out his sketchpad and made drawings of demonstrations he attended. Also logged were slogans on his clothes and details of his appearance, such as "clean-shaven".

His legal action threatens to deal another blow to the secretive National Public Order Intelligence Unit, which has been covertly monitoring protesters since 1999.

The unit has recorded the activities of thousands of campaigners on a nationwide database. Defeat in the court case would put pressure on it to delete details of activists from the database.

A similar case in 2009 compelled the Metropolitan police to remove 40% of the photographs it held on a database of protesters after the court of appeal ruled that the force had unlawfully retained an image of an anti-war campaigner, Andrew Wood.

Over the past year the unit has been engulfed in criticism after the unmasking of Mark Kennedy, the undercover police officer who infiltrated the environmental movement for seven years. Last week, an official police watchdog criticised the conduct of Kennedy and his superiors at the unit.

The watchdog, Sir Dennis O'Connor of Her Majesty's Inspectorate of Constabulary, said the unit was unlikely to be justified in recording and retaining intelligence "in a number of cases".

Kennedy and other undercover police officers gathered secret intelligence that was fed into the database. The unit has also drawn on information from uniformed surveillance teams and telephone taps, informants in political groups, and companies to amass the secret files.

Catt's lawyers will argue that he should be permanently taken off the database as he is "committed to protesting through entirely peaceful means".

In legal papers, Catt has described how the police have recorded the political aims of the demonstrations he was at between 2005 and 2009. Some of the files contained "highly personalised" information about his appearance and "hearsay evidence and police officers' opinions", he has argued.

He highlighted how police recorded his arrival at one demonstration and that he "sat on a folding chair and appeared to be sketching".

At another demonstration, in March 2006, police noted that "John CATT arrived in his white Citroen Berlingo van. He removed several banners for the protesters to use and at the completion of the demo returned the same to the van. He was using his drawing pad to sketch a picture of the protest and the police presence."

On another occasion, police wrote that he was on a demonstration against Guantánamo Bay in September 2005, adding: "John CATT was seen wearing a Free Omar T-shirt, he was clean-shaven … John CATT was very quiet and was holding a board with orange people on it."

Police chiefs have argued that they are legally justified in maintaining the files on Catt, describing the surveillance as "minor". Lawyers for the police say Catt has been taking part in a campaign to close down a Brighton arms factory owned by the US firm EDO MBM Technology. This is a "campaign of illegality designed to pressurise EDO to cease its lawful business" and has led to a series of convictions of campaigners, according to the police.

Police say the surveillance of Catt is necessary because "his voluntary association at the Smash EDO protests forms part of a far wider picture of information which it is necessary for the police to continue to monitor in order to plan to maintain the peace, minimise the risks of criminal offending and adequately to detect and prosecute offenders".