Environmental activists who were arrested before they could execute a planned shutdown of a coal-fired power station near Nottingham in April last year were today convicted of conspiracy to commit aggravated trespass.

The 20 campaigners were among 114 detained on Easter Monday 2009 in the largest pre-emptive police operation against environmentalists ever mounted in the UK. They were arrested on suspicion of planning to occupy and close down E.ON's operations at Ratcliffe-on-Soar for a week. The 14 men and six women will be sentenced at Nottingham crown court on Friday. The maximum penalty for the offence is three months' imprisonment, a £2,500 fine or both. The police operation to thwart their action cost £300,000.

The campaigners are members of a network that developed out of Climate Camp protests. They admitted planning to shut down the power station but denied the charge they faced, using a defence of "necessity". They argued they were acting to prevent the greater crimes of death and serious injury caused by climate change, and called expert witnesses to support them – including the Nasa climate scientist James Hansen.

The jury had begun considering its verdict on Friday. Speaking afterwards, Clare Whitney, one of the campaigners, said: "During this trial we have heard from people on the frontline of our changing climate, and from a company that is still burning the most dirty form of fossil fuel for their economic benefit. These worlds are not compatible. Taking action is not an issue of moral righteousness but an act of self-defence. If we're to stand a chance of avoiding irreversible climate change we've got to realise that to bring about a better world we'll need to do it ourselves."

Another of those convicted, Chris Kitchen, said, "We are in solidarity with all those around the world fighting for climate and social justice. Together we need to stop the root causes of climate change, we need to stop profit being put before people. It's big business and politicians that are the real criminals and we will not stand by as we are robbed of our future."

Dan Glass, another defendant in the case, said: "This ruling won't stop emissions. But the huge support we have received from the people of Nottingham and internationally, does demonstrate that public opinion is increasingly turning against the liberties that governments are taking with our future."

The campaigners claimed that had they succeeded, they would have prevented the emission of 150,000 tonnes of CO2.

E.ON declined to comment on the verdict. Six people who said they had not decided whether to take part in the protest when they were arrested face a separate trial next month.

In his evidence, Hansen suggested the actions of the campaigners were understandable. He told the court last month: "The fact that we continue to burn more coal and build more coal plants shows governments are not telling the truth.

"If they are saying they understand the climate problem but will continue to burn coal it's easy for me to understand that young people get upset, because they know governments are lying or kidding themselves."

Nottinghamshire Police welcomed the verdict. Superintendent Adrian Pearson, who led the investigation, said it had showed "that these individuals were determined to commit offences that would result in them not only trespassing on private land, but also damaging property and potentially endangering the lives of others. These actions would have had a major impact on people living not only in Nottinghamshire, but across the East Midlands."

"Nottinghamshire Police acted swiftly to successfully disrupt their plans in what was a massive operation for the force, costing around £300,000. However, if these people had been successful, the cost would have been much higher, not only to the police, but also in terms of disruption to the public and damage to the power station," he added.

The conviction came as a blow to environmental campaigners. Greenpeace members had successfully argued "lawful excuse" as a defence after another power station protest. Six were acquitted of causing criminal damage at Kingsnorth power station in Kent in 2007 when they were tried the following year.

During the case, Felicity Gerry, for the prosecution, had suggested that direct action could "disengage ordinary people" such as the members of the jury. They might see activists as "a bit quirky and kooky and not necessarily mainstream".

She asked one defendant why the group had not spent £15,000 they set aside for the planned occupation on a celebrity such as Cheryl Cole to front campaigns to tackle climate change.

She also suggested to Sarah Shoraka, biodiversity campaigner with Greenpeace, from North London, that the Meat-Free Monday campaign to cut down on meat consumption, promoted by Coldplay's Chris Martin and Sir Paul McCartney, was "quite an effective campaign because some celebrities are involved". The activists could have tried something similar. "Why did you not just do 'Turn off Tuesday' or 'Switch off Saturday'?"

Shoraka answered that the action was not a publicity stunt. "I was very pessimistic at the time. I did not see there was any political will to go anywhere near tackling the problem."

Ed Rees QC, for the defence, had earlier said: "The prosecution suggests that it [the protest] was a laugh and a bit of a jolly and was for fun."

Much of Hansen's evidence revolved around the need to avoid reaching a tipping point where climate change would spiral outside of control. One key "amplifying feedback" was the melting of ice-caps, he said, which cause the earth to warm more quickly because oceans absorb heat rather than reflect it like snow and ice. If melting reached a certain rate, it would be irreversible. "You do not want to reach a point where you begin to get collapse (of ice sheets) and rapid change. If you reach that point you have gone too far and it will be out of humanity's control."

A statement issued on behalf of all those on trial, asserted that, as UN climate talks finished in Cancún, Mexico, with no legally binding framework on reducing emissions, "the British legal system is still upholding business as usual"."