Last weekend I invited one of my closest friends over for dinner. We chewed over the usual unremarkable topics: The X Factor, future plans, idle gossip, the new Bond film. And then the conversation turned to something less unremarkable; something most people will never talk about with their friends. What if none of the memories we share, the secrets we've told each other, or the histories we've disclosed to one another were real? What if everything we knew about each other was based on a lie, so that one of us could extract information from the other that would eventually be used against them? It was such a distressing thought that we changed the subject almost immediately.

My friend is an activist. I know quite a lot of activists. I have been one for some time. I don't know any who have hurt another human being. I don't know any who are a danger to society. Most of the activists I know engage in the odd nonviolent demonstration, like the protesters at Fortnum & Mason, some of whose guilty verdicts were overturned yesterday, or the people halfway up West Burton power station.

Despite the peaceful nature of their actions, the simple act of protesting means that activists' lives sometimes resemble that of Tony Soprano. Surveillance, police intimidation and undercover officers are routine hazards they must negotiate. As one environmental campaigner who has come into contact with undercover officers puts it: "You don't have to be self-important to suspect you're the victim of state surveillance. If you're politically active, it's simply a fact of life."

Occasionally we read stories of undercover officers, or police intimidating campaigners in their homes or sending threatening letters; but rarely do we talk about the psychological toll this takes – that a feeling of constantly being watched is an invariable factor in the lives of people who take part in protest. Often activists are depicted as being like the mafia – part of an underground coterie that is somehow separated from ordinary life. It's a depiction that makes constant surveillance seem acceptable, perhaps even justified. But activists are just ordinary people who work, watch TV, and drink too much at the weekend, just like anybody else.

After police unlawfully visited me at my home following a protest to tell me they were "watching me", I still jump every time my doorbell rings. More seriously, I have female friends who are reticent about relationships in case the men they are sleeping with are not who they say they are. One person whose campaign group was infiltrated told me the reason it often takes activists a while to spot undercover officers is denial. "It's just too horrible to contemplate that every memory you've shared with that friend has been a lie," she said.

Consistently being treated like a criminal, even when you're not doing anything illegal, excludes you from normal society – as though that's something you have to give up if you want to act on your political beliefs. The officers who allegedly fabricated evidence at the battle of Orgreave or those who slept with campaigners, to gain their trust, have never been disciplined. The message is clear: if you participate in activism nothing is off limits and you're on your own.

Last April the foreign secretary, William Hague, called for Syria to "respect basic and universal human rights to freedoms of expression and assembly". What does the intrusive surveillance of activists mean for a society that champions freedom of expression to the world? How can we justify extending the net of surveillance so widely, unsettling the lives of so many, and apparently for so little? The truth is that enduring this type of psychological harassment – for that's what it amounts to – is not the domain of paranoid fantasists or mobsters. It is a routine problem for anyone taking part in even the most innocuous forms of dissent.

That conversation I had with my friend over dinner was not outlandish; it was almost a rite of passage. It is the sort of conversation that activists must expect to have in a society such as ours, which spies so relentlessly on its own citizens.