The founder of Extinction Rebellion has been cleared by a jury of all charges relating to a protest against fossil fuels in what campaigners say is a historic moment for the climate justice movement.

Roger Hallam, 52, did not deny criminal damage worth £7,000 in an action to urge Kings College, London to divest from fossil fuels.

He sprayed the walls with the words “divest from oil and gas” in protest against the institution’s fossil fuel investments.

But on Thursday after a three day trial at Southwark crown court, Hallam and another activist were cleared of all charges. They represented themselves and argued to the jury in their defence that their actions were a proportionate response to the climate crisis.

On Thursday the jury acquitted Hallam of two charges of criminal damage and his fellow activist David Durant, 25, of one charge of criminal damage.

Hallam said: ‘We are extremely grateful to the jury for following common sense … ordinary people, unlike the judiciary, are able to see the broader picture.”

Speaking outside court, Durant said: “We sat in the court, we watched paint dry for three days on a ridiculous charge and the jury returned the common sense verdict of not guilty.

“Chalk on the wall is obviously less important than the impending catastrophe for the planet.”

Q&A What is Extinction Rebellion? Show Extinction Rebellion is a protest group that uses non-violent civil disobedience to campaign on environmental issues. Launched in October 2018, with an assembly at Parliament Square to announce a 'declaration of rebellion' against the UK Government, the group has staged regular demonstrations against current environmental policies. More than 1,000 activists were arrested in April 2019 after protesters occupied four sites across London, as well as blocking roads, disrupting a railway line and conducting a protest at Heathrow. Other demonstrations have included a semi-naked protest inside the House of Commons and blockading streets in London, Cardiff, Leeds, Bristol and Glasgow. The group says climate breakdown threatens all life on Earth, and so it is rebelling against politicians who “have failed us”, to provoke radical change that will stave off a climate emergency. The movement has become global with groups set up in countries include the US, Spain, Australia, South Africa and India. Martin Belam Photograph: Anadolu Agency/Anadolu

When university security guards intervened on 19 January 2017, Hallam handed them homegrown salad including red mustard leaves, rocket and rainbow chard as a gift.

Hallam and Durant were arrested several days later, on 1 February, when they spray painted the internal walls of the university’s Great Hall.

Tim Crosland, the director of charity Plan B Earth, said the verdict was a historic moment for the climate protest movement.” He added: “Mr Hallam and Mr Durant did not deny they had caused the damage; but argued that their intervention was a proportionate response to the climate crisis.”

Adam Loxley, former head of security at the university, described how four columns at the front of the listed 1960s building had been defaced with the words “Divest from oil and gas”, “Now!” and “Out of time”.

In a video shown to the jury, Hallam said: ‘This is not about polar bears, this is about mass starvation. It is a total emergency, if we do not take drastic action, our civilisation will soon collapse.”

In a second video shown to the jury, Durant can be heard saying: “You guys seem very concerned about the fact that we’re potentially damaging your building but not by the fact that King’s is damaging the planet.”

A security guard told the court fellow protesters set off smoke grenades, which triggered the fire alarms. The Great Hall had to be evacuated as a private function was under way at the venue.

Giving evidence, Hallam explained that he gave security guards his homegrown salad and said: “In my polytunnels I grow salads. A big part of Gandhi and civil disturbance is to give gifts to people who oppose you as a sign of good faith.”

Hallam, who is studying for a PhD in civil disobedience at King’s College, explained that his academic studies influenced his decision to deface the university. He said: “My approach and my research is how we can update the work of Gandhi and Martin Luther King in a modern context.

“Everything we did in this campaign was organised with the specific intention to maximise respect, both between ourselves the students at King’s College and the college authorities.”

Hallam argued his actions were lawful because there was an exemption in the Criminal Damage Act that permits damage if it protects others’ property.

The prosecutor, John Hulme, had told jurors there was “no legitimate basis for applying this spray, even if the defendants did not agree with the policy of the college”.

He said the pair had used soluble paint but it could still be considered criminal damage under the law.

“Also, it is a fact that some £7,000 was spent by the college to wash away the spray that had been applied,” he said.

The judge ruled during the case the issue of climate change was “irrelevant” to the case. He said it was not a case about the issue of climate crisis, but about damaging property and whether the defendants had a lawful excuse.

He said he would not allow the trial to be “sidelined into the issues”.

The decision will be watched with interest by protesters who were charged after taking part in the Extinction Rebellion protests that disrupted motor traffic in central London and other cities last month, many of whose cases will be heard in the coming weeks.