The chief constable of Thames Valley police has apologised after her force placed peace campaigners attending the screening of a documentary about nuclear weapons under surveillance, fearing they were intent on protest.

Guests who went to Reading International Solidarity Centre in July to see Countdown to Zero, with an introduction by the town's mayor, complained that two officers were posted outside to record the details of those entering.

The guests said that the officers, who were joined by members of the Ministry of Defence police, were seen making notes about individuals and car registration numbers. They also suggested officers were filming individuals using cameras mounted on their tunics, a claim denied by Thames Valley police.

The revelation has triggered concerns that police are subjecting peace campaigners to the type of surveillance endured by environmental protesters in recent years. The heavy-handed tactics drew anger from Peter Burt, director of the Nuclear Information Service (NIS), which organised the July screening. The film features interviews with global leaders like Tony Blair, Mikhail Gorbachev and former US secretary of state James Baker.

Burt branded the surveillance of "non-violent" political activity "a hallmark of totalitarian regimes that should never have any place in policing in this country". In a letter to Sara Thornton, the force's chief constable, Burt said: "NIS's role is educating and informing, and we are not a campaign organisation. The event had no protest theme whatsoever to justify the presence of the police."

He said that after confronting the officers they informed him they had been sent to "watch out for certain people". The officers declined to elaborate.

In her letter of apology, Thornton said the officers were deployed as part of a wider police operation that weekend in relation to potential protest activity in the West Berkshire area.

"Officers were also tasked with conducting general observations in the Reading area to help us ascertain the likelihood of any protest activity," Thornton wrote. "Your event was just one of many locations the officers patrolled during the day but with hindsight I would have preferred for them not to have attended the event." Thornton said Thames Valley officers "were not writing notes about everyone attending the event".

However, the NIS has now written to the Ministry of Defence police to ask whether its officers were responsible for filming the audience.

Thornton's letter stated: "I cannot account for any activity undertaken by the MDP officers as they were not taken by the chief inspector overseeing the operation for Thames Valley police."

Burt said: "We're grateful to the chief constable for her apology, and hope that lessons will be learned by the police in the light of the matter. It's an unfortunate fact that, while police officers were busy spying on film-goers, crimes were being committed elsewhere in the Thames Valley, and the police seemed to have got their priorities wrong on this occasion."