Police have carried out what is thought to be the biggest pre-emptive raid on environmental campaigners in British history, arresting 114 people believed to be planning direct action at a coal-fired power station.

The arrests - for conspiracy to commit criminal damage and aggravated trespass - come amid growing concern among protesters about increased police surveillance and infiltration by informers.

Police said the raid on a school in Nottingham was made just after midnight yesterday, and was linked to a planned protest, thought to be at nearby Ratcliffe-on-Soar power station.

Nottinghamshire police said action had been taken because "in view of specialist equipment recovered by police, those arrested posed a serious threat to the safe running of the site".

The mass arrest, involving three police forces, prompted renewed concern about the tactics of officers policing environmental protests, particularly over expansion of airports and coal power.

Last night campaigners said police were photographing and stopping people entering and leaving public meetings and the offices of the lobby group Greenpeace.

Policing of the recent G20 protests has also come under scrutiny after video evidence suggested the newspaper vendor Ian Tomlinson died minutes after being pushed from behind by a police officer.

Caroline Lucas, leader of the Green party, said that with no charges more than 12 hours after the arrests, "confidence in policing of protests like this has just about hit rock bottom.

"Peaceful protest is a civil liberty we need to be upheld, even more in the context of the lack of government action on climate change. We have tried all the usual channels."

Shami Chakrabarti, director of Liberty, said: "In the light of the policing of the G20 protests, people up and down the country will want to be confident that there was evidence of a real conspiracy to commit criminal damage by those arrested and that this was not just an attempt by the police to disrupt perfectly legitimate protest."

No group had last night claimed responsibility for the alleged demonstration, which fellow activists said was because charges of conspiracy could lead to a jury trial and possible jail sentence.

Emails setting out planned action could be used by police to prove conspiracy to commit aggravated trespass and criminal damage, as could any equipment or documents found during the arrests.

No formal details of the proposed action have been released, but campaign veterans speculated that demonstrators may have planned to chain themselves to the conveyor belts taking coal into the power plant in order to stop the generators when fuel ran out. The tactic has been used at both Ratcliffe and a coal plant at Kingsnorth, Kent, both owned by the utility company E.ON.

There was speculation that the arrests in Nottingham followed a tip-off from a police informer.

Other campaigners said the size of the protest group made it vulnerable to routine police surveillance.

There have been a growing number of demonstrations and "direct actions" by campaigners angry about government plans for a new generation of coal-fired power stations.

The secretive nature of the alleged protest plan is similar to actions organised by the Climate Camp network, a loose affiliation of groups, but nobody from the movement could be contacted yesterday.

Last month a Guardian investigation revealed police were targeting thousands of political campaigners in surveillance operations at events including the Climate Camp, and storing their details on a database for at least seven years.

Nottinghamshire police said the 114 people arrested were from across the country. "There were no injuries during the arrests and the police investigation is ongoing." Some of those arrested were taken to police cells in Leicestershire and Derbyshire.

E.ON said: "We can confirm that Ratcliffe power station was the planned target of an organised protest during the early hours of this morning. While we understand that everyone has a right to protest peacefully and lawfully ... we will be assisting the police with their investigations into what could have been a very dangerous and irresponsible attempt to disrupt an operational power plant."

UK climate protests

March 2009 - Guardian investigation reveals police targeting thousands of environmental campaigners and journalists at demonstrations

September 2008 - Jury acquits protestors accused of causing £30m criminal damage by scaling Kingsnorth power station who argued they were protecting the world

August 2008 - Thousands join the third Climate Camp near Kingsnorth in Kent in protest against coal power

June 2008 - Protestors hijack a coal train in Yorkshire. 29 people face trial

April 2008 - Actions around the country for "Fossil Fools Day" include a blockade of E.On's offices in Nottingham

April 2007 - Activists from Eastside Climate Action break into Ratcliffe-on-Soar power station and chain themselves to buildings; 11 were arrested

August 2006 - About 600 people turn up for the first Climate Camp at Drax power station, the UK's biggest single source of carbon dioxide emissions