Footage shot by police shows how their officers monitored campaigners and the media at last August's climate camp demonstration in Kent guardian.co.uk/Jason N. Parkinson

Kent police accepted last night they should not have placed journalists under surveillance after a Guardian investigation revealed members of the press were monitored and followed at last year's climate camp demonstration.

Secret footage shot by two police surveillance officers during the protest, obtained by the Guardian and broadcast online over the weekend, confirmed officers have been monitoring journalists at protests. Senior officers had previously denied journalists had become the target of surveillance units.

The footage showed that while officers had been asked to monitor protesters against the Kingsnorth coal-fired power station, they showed particular interest in journalists.

An ITV news crew, a Sky News cameraman and several photographers were among members of the press placed under surveillance as they left the camp in August. Later in the day journalists were followed by another surveillance unit to a McDonald's restaurant where police filmed them.

Allyn Thomas, Kent's assistant chief constable who oversaw the operation, said the force "fully accept the right to protest as part of our democracy", but the force had received information that protesters intended to shut down the power station.

"As part of our operation overt filming of protesters was carried out as a means of identifying anyone who subsequently attempted to break into the power station. The situation on the ground was complex with more than 1,000 protesters, together with journalists and camera crews, in a confined area. We accept that police should not have filmed legitimate journalists or camera crews, however it was a difficult task in these circumstances to clearly identify them." All the journalists followed by the surveillance unit were carrying camera equipment and had produced press cards, the official identification of bona fide journalists.

But Thomas said there was "an issue" with officers who had failed to understand press cards, which are endorsed by national police bodies as the official identification for news gatherers.

"We acted on this and have incorporated training about this issue in our guidance for future operations," he said.

In a separate development it also emerged last night that the Metropolitan police have been asked to clarify evidence they submitted to the court of appeal about the use of the tactic, following the Guardian's disclosures.

Lawyers at the human rights watchdog Liberty representing Andrew Wood, a former press officer for the Campaign Against Arms Trade, who is challenging the legality of the Met surveillance in a judicial review, have asked the force to clarify its submissions to the courts.

In its evidence the Met said surveillance images of protesters were placed on CD-Roms and stored in a warehouse. Images are usually kept only for the use by officers of the public order branch, it said.

But the force appears not to have disclosed that - when officers deem it justified - surveillance material about protesters is transfered to Crimint, the general database used daily by all police staff to catalogue criminal intelligence.

Lawyers believe the transfer makes it more likely the technique is in violation of article 8 of the Human Rights Act.

Wood, from Oxford, was targeted by a Met surveillance unit as he emerged from London's Millennium Hotel in April 2005, where he had attended the annual general meeting of a company that ran a trade fair for the arms industry. He had gained access to the meeting by lawfully purchasing a share in the company.

Expecting a demonstration and possible unlawful activity by campaigners, the Met deployed three surveillance teams. Wood was photographed with other campaigners as he emerged from the meeting and then followed by police to a tube station. His claim that the police's taking of the photograph - after he lawfully attended a meeting - breached his rights under the European convention on human rights was dismissed by the high court last year.

A judgment by the court of appeal is expected soon, but may be delayed while the Met clarifies its position.

Shami Chakrabarti, director of Liberty, said: "The police need to distinguish between crime and democratic dissent or they will find themselves on the receiving end of the law."

A growing tactic

Overt surveillance was first used by police to monitor football hooligans and animal rights activists in the late 1990s. Pioneered by the Metropolitan police, the tactic has been expanded to cover all forms of political demonstrations and meetings, with surveillance units regularly deployed across the country. Those photographing or filming protesters are specially trained civilians or police officers, and are used to monitor movements to help the deployment of resources, or as gatherers of potential evidence. Indications suggest such surveillance is to increase. The Association of Chief Police Officers has commissioned the National Police Improvements Agency to establish a national standard for the technique.