Four campaigners have lodged a complaint with an official watchdog, alleging that they felt intimidated by covert police officers who were trying to persuade them to spy on their political colleagues.

They say that coercive and at times repeated police approaches caused them to abandon their political campaigning, or left them stressed and paranoid.

They also say that the clandestine manoeuvres to spy on students, environmentalists, anti-fascists and other campaigners erode free speech and the freedom to protest.

One of the quartet, a 23-year-old single mother, said she stopped campaigning against racism after police threatened to prosecute her if she told anyone, including her mother, about the attempt to recruit her as an informer.

She said she was left "feeling violated, isolated, vulnerable and paranoid" as she worried that any prosecution would imperil her young son, her place at university and her opportunities to work in the future. She said that at one point, police were calling her daily and sometimes twice a day to try to recruit her.

Their lawyer, Jules Carey from the civil liberties firm Bindmans, said : "It is not only right but imperative for the health of our democracy that police forces are challenged when they use spy tactics that undermine freedom of speech and rights to protest without the strongest of justifications."

Their submission to the Independent Police Complaints Commission (IPCC) comes as controversy increases about the police's undercover infiltration of political groups that campaigners insist are engaging in legitimate protest.

Undercover police have been criticised for spying on grieving families and using sexual relationships with women to gather information.

Police are believed to be running hundreds of informers in political groups across the country, but little is known about them.

The IPCC complaint centres on a unit within Cambridgeshire police that tried, without success, to recruit the four campaigners who do not want to be named in public.

Cambridgeshire police did not wish to comment before a response from the IPCC. The force has previously accepted that it tried to enlist the four, but has denied, without going into detail, some of the conduct alleged by them.

It said that its officers had used "covert tactics to gather intelligence, in accordance with the law, to assist in the prevention and detection of criminal activity".

One of the activists, an environmentalist in his 20s, had covertly recorded a police officer trying to persuade him to pass on information in return for cash about the political activities of Cambridge students, Unite Against Fascism and UK Uncut, the anti-tax avoidance campaign.

The secret footage, broadcast by the Guardian last November, revealed how the police wanted him to name students who were going on protests, describe the vehicles they used and identify leaders. In the complaint, he describes how the attempted recruitment made him stressed, and left him worried that he was "marked out" by the police because he had refused to sign up as an informant.

A third campaigner, also an environmentalist, said he had been left "paranoid and mistrustful of people" after police tried to persuade him to spy on his colleagues.

He said that a police officer made two unannounced visits to his home and later followed him and his four-year-old daughter to a supermarket where he tried to thrust an envelope stuffed with cash into his hands.

He said he felt "intimidated as he felt he had been deliberately targeted because of his political activities".

The fourth campaigner, a Cambridge University student, said he had called the police to report suspected burglars.

He said police invited him to the station to discuss his suspicions, but during the meeting they tried to recruit him as a paid informer to spy on leftwing students.

He says he "consequently suffered from paranoia and completely dropped out of protest activity in Cambridge".

The four complain that the police have intruded into their lives and used personal information to lure them into becoming part of their espionage operation.