Police sought to launch a secret operation to spy on the political activities of students at Cambridge University, a covertly recorded film reveals.

An officer monitoring political campaigners attempted to persuade an activist in his 20s to become an informant and feed him information about students and other protesters in return for money.

But instead the activist wore a hidden camera to record a meeting with the officer and expose the surveillance of undergraduates and others at the 800-year-old institution.

The officer, who is part of a covert unit, is filmed saying the police need informants like him to collect information about student protests as it is "impossible" to infiltrate their own officers into the university.

The Guardian is not disclosing the name of the Cambridgeshire officer and will call him Peter Smith. He asks the man who he is trying to recruit to target "student-union type stuff" and says that would be of interest because "the things they discuss can have an impact on community issues".

Smith wanted the activist to name students who were going on protests, list the vehicles they travelled in to demonstrations, and identify leaders of protests. He also asked the activist to search Facebook for the latest information about protests that were being planned.

The other proposed targets of the surveillance include UK Uncut, the campaign against tax avoidance and government cuts, Unite Against Fascism and environmentalists. The Cambridgeshire police initially insisted that there were implications for "national security" but later dropped this argument when challenged.

At another point, the activist asked whether a group known as Cambridge Defend Education, which has protested against tuition fees and education cuts, would be of interest. Smith replied: "That's the sort of thing that we would be looking for. Again, basic sort of stuff. It's all the internet. When they have meetings and they are discussing what they are going to do, that's when we'll say: 'Will you go along?'"

Cambridge Defend Education describes itself as being "mostly students and academics from Cambridge University".

Rachel Wenstone, deputy president of the National Union of Students, said: "This is yet another example of the questionable tactics that undercover police officers have taken in recent years to infiltrate campaign groups and extract information."

Julian Huppert, the Liberal Democrat MP for Cambridge, tweeted: "I'm shocked by this – seems wholly inappropriate." Cambridge University did not comment, saying it was a matter for the police.

Cambridgeshire police said: "Officers use covert tactics to gather intelligence, in accordance with the law, to assist in the prevention and detection of criminal activity."

The disclosures follow prolonged criticism of the police over their secret deployment of long-term undercover officers in political groups since 1968. Police chiefs have been accused of unjustifiably infiltrating and disrupting political groups that use non-violent methods to promote their aims.

Another technique for gathering intelligence on campaigners has been to convince activists to become paid informants and pass on details of future protests and prominent campaigners. The number of informants in political groups, according to police sources, runs into the hundreds.

The covert film sheds light on the rarely visible world of informants, illuminating how the police recruit and task them. The activist, who does not want to be named and has been given the pseudonym John Armstrong, was rung on his mobile out of blue at the beginning of October by the police officer.

Smith said he worked for the police and asked him if he was willing to come to a police station in Cambridge to help him with a matter that he did not disclose.

According to Armstrong, Smith had chosen him because he had been active in environmental and anti-nuclear groups and had been arrested three times on demonstrations, although not charged. He has also lived in Cambridge for many years.

Afterwards, Armstrong contacted the Guardian as he did not want to become an informant. He agreed to wear a concealed camera to record the contents of his second meeting with Smith.

During this meeting, Smith suggested that he wanted Armstrong to start by providing information about local groups in Cambridge, before progressing on to national campaigns.

"Let's keep it small, you know little things that go on, little meetings that happen where they are going to discuss different issues in Cambridge, whether it be, such as at the university or those sorts of things," the officer is recorded as saying. When Armstrong said he had been involved in a student-organised occupation of Cambridge University in a protest against tuition fees three years ago and asked if Smith would have been interested in that, Smith said yes. "Again, it's those sorts of things. You know, what is the feeling of people, if you are inside."

The young man then asked if it would have been difficult for the police to send their own officers into the occupation, to which Smith replied: "We can't do it. It's impossible. That's why we need to work with people." Armstrong has not been a student at Cambridge, although many of his friends are at the university.

When contacted by the Guardian, a Cambridgeshire police spokesperson said: "Officers use covert tactics to gather intelligence, in accordance with the law, to assist in the prevention and detection of criminal activity." They declined to give any details of the unit Smith works for.

Smith outlined what information Armstrong would be required to slip him. "It will be a case of you going to meetings, say, I don't know, UK Uncut, student … something like that, how many people were there, who was the main speaker, who was giving the talks, what was your assessment of the talk, was it a case of – were they trying to cause problems or were they trying to help people, you know, those sort of things."

Smith also said he wanted Armstrong to collect information about Cambridge campaigners who were planning to go to protests in other parts of the country. "That's where the names come in. Because what I will want to know is – OK, who's going, do they plan on a peaceful protest which is absolutely fine, how they are going to go, as in what vehicles they are going to use, index numbers."

He goes on to say: "So you will tell me, for example, there's 50 people going from Cambridge University, these are the vehicles they are travelling in and they are going as a peaceful protest?"

Smith outlined how the information gathered by Armstrong would be funnelled to the police officers in charge of policing the demonstration: "The reason I am asking those questions is because it gives the officers or whoever's looking after it on that side of things, as in at the protest, an idea of how many people are going to attend, where they are coming from, how many vehicles are going to turn up, so they can put measures in place to keep them off the road and things. It's not because we want to target people and round them all up and arrest them."

Smith also suggested that Armstrong use Facebook to find information about groups, adding: "It is easier to ask people like yourself to give us updates … It's all about us doing things legally … We don't hack into people's accounts so then we would ask you for updates."

The officer also suggested the man he hoped to recruit would be paid expenses or other sums. "You might go to a UK Uncut or Unite Against Fascism meeting one evening, you might get say £30 just for your time and effort for doing that. That's the sort of thing you are looking at."

As Smith sought to convince Armstrong to sign up, he also advised him not to "think too deeply" about informing on his fellow campaigners as he might "tie himself up in knots".