A pensioner and his daughter said they were victims of a “police state” after an anti-terror unit pulled over their car because it had been spotted at a political protest.
The pair’s experience is expected to provoke a civil liberties debate when they feature in a major TV documentary tonight.
They say surveillance coupled with anti-terror powers mean innocent people are criminalised merely for appearing on police databases.
John Catt, 84, and his daughter Linda, 49, are appearing tonight in “Who’s Watching You?” the first of three BBC2 documentaries about surveillance.
They had a marker placed against their car on the Police National Computer by Sussex Police after attending three anti-arms demonstrations against EDO MBM Technology in Moulsecoomb.
Months later an automatic number plate recognition (ANPR) camera in central London flagged them up as “of interest to public order unit Sussex”.
They were pulled over and searched by a police anti-terrorism unit - and threatened with arrest under the Terrorism Act if they did not explain where they were going. Neither had a criminal record.
The ANPR cameras read number plates and record where the time and date where they have been spotted.
They are chiefly used to track cars which have been reported stolen, or are implicated in a crime.
Their use has become increasingly widespread and a project is under way to create a central national database and adapt thousands of CCTV cameras to use the technology.
Sussex Police alone record 1.2 million car journeys a day using ANPR. Senior officers regard it as a vital tool in fighting crime.
The Catts were pulled over on July 31, 2005 - days after a second co-ordinated terrorism attack on London failed.
The note which triggered the ANPR alert had been placed against their car number plate in March that year.
A Sussex Police spokesman refused to confirm or deny whether the police marker still exists against their vehicle.
Miss Catt, a lawyer, said: “I was very angry and I am still very angry because I tried to find out from Sussex Police whether our vehicle is still marked but they refused.”
“I feel like we have been criminalised and I also worry because if I want to go abroad in that vehicle, with all the databases linked up, I could be stopped in Europe.
“It has infringed our civil liberties and this is why we are trying to highlight the issue.”
Miss Catt made a formal complaint to Sussex Police after they were stopped.
Their complaint was not upheld by the Independent Police Complaints Commission and the City of London Police.
They were told use of the marker was “proportionate and appropriate”.
Miss Catt and her father say that the use of the police marker has been misused against them for simply exercising their democratic right to protest against the multi-billion pounds arms trade.
Mr Catt hit the headlines at Brighton's Labour Party Conference in 2005 when he was searched under the Terrorism Act on his way to campaign for the release of Guantanamo Bay prisoner Omar Deghayes.
He said: “That our participation in peaceful protest outside an arms factory led to our arbitrary stop-check for terrorist activities many miles away by another force is a very disturbing development of the 'police state'.”
Miss Catt said she would like to see the ANPR regulated so that innocent people are not targeted.
She said: “The fact that it isn't regulated means it could have grave implications for anybody.
“You will not know there is a marker on your car until you are stopped by police.
“I appealed to the IPCC when my complaint was not upheld and they agreed with the police. There is no legal remedy for me at the moment.”
Corinna Ferguson, legal officer at Liberty, which campaigns to protect civil liberties and promote human rights, said: "Quite apart from being yet another example of apparent misuse of Section 44 of the Terrorism Act, this type of response to lawful protest fails to respect privacy rights and will further undermine public trust in the police.”
Guy Herbert, general secretary of campaign group NO2ID, said experiences like this would become a common occurrence if the expansion of ANPR surveillance continues.
He said: "That a lawyer and her octogenarian father are treated like public enemy number one is extraordinary enough, but if police seek patterns in the vast amount of surveillance data they are collecting then this is precisely what will happen.
"If you are convinced there are patterns to be found, then any 'data mirage' such as this can be mistaken for proof.
"The Association of Chief Police Officers have made it perfectly clear that they aim to track every car journey throughout the country and store that data for two years - even with no statutory powers to do so.
“With that amount of information, the same kind of confusion will lead thousands of other innocent people to be hauled up and harassed by officials who cannot see the data woods for the data trees."