I was sent the now notorious "police spotter card" through the post. It's an official laminated card for "police eyes only" and labelled as coming from "CO11 Public Order Intelligence Unit". The card contained the photographs of 24 anti-arms trade protesters, unnamed but lettered A to X. My picture appeared as photo H. You can imagine my reaction at finding I was the subject of a secret police surveillance process … I was delighted. I phoned my agent and told him I was suspect H. He replied: "Next year we'll get you top billing … suspect A."

The Metropolitan police circulated the card specifically for the Docklands biannual arms fair in London to help its officers identify "people at specific events who may instigate offences or disorder". Which is such a flattering quote I am thinking of having it on my next tour poster. While being wanted outside the arms fair, I was legitimately inside researching a book on the subject, and uncovered four companies illegally promoting "banned" torture equipment. Questions were later asked in the Commons as to why HM Revenue & Customs and the police didn't spot it. Though, in fairness, none of the torture traders featured on the spotter card.

What exactly was I doing that was so awfully wrong as to merit this attention? Today's Guardian revelations of three secret police units goes some way to explain the targeting of protesters and raises worrying questions. The job of these units is to spy on protesters, and collate and circulate information about them. Protesters – or, as the police call them, "domestic extremists" – are the new "reds under the bed".

Many of those targeted by the police have committed no crime and are guilty only of non-violent direct action. So it is worth reminding ourselves that protest is legal. Sorry if this sounds obvious, but you might have gained the impression that if three police units are spying on and targeting thousands, then those people must be up to something illegal.

The very phrase "domestic extremist" defines protesters in the eyes of the police as the problem, the enemy. Spying on entire groups and organisations, and targeting the innocent, undermines not only our rights but the law – frightfully silly of me to drag this into an argument about policing, I know.

Protest is part of the democratic process. It wasn't the goodwill of politicians that led them to cancel developing countries' debt, but the protests and campaigning of millions of ordinary people around the world. The political leaders were merely the rubber stamp in the democratic process. Thus any targeting and treatment of demonstrators (at the G20 for example) that creates a "chilling effect" – deterring those who may wish to exercise their right to protest – is profoundly undemocratic.

No police, secret or otherwise, should operate without proper accountability. So how are these three units accountable? Who has access to the databases? How long does information remain in the system? What effect could it have on travel and future employment of those targeted? How closely do these units work with corporate private investigators, and does the flow of information go both ways? Do the police target strikers?

A police spokesman has said that anyone who finds themselves on a database "should not worry at all". When a spokesman for the three secret units will not disclose a breakdown of their budgets, and two of the three will not even name who heads their operations (even MI6 gave us an initial, for God's sake), then the words "should not worry at all" are meaningless. Indeed, when the police admit that someone could end up on a secret police database merely for attending a demonstration, it is exactly the time to worry.