Three Extinction Rebellion activists involved in protests in central London in April have been convicted of public order offences at a trial which heard a message of support for them from the shadow chancellor, John McDonnell.

The men were among more than 1,000 people arrested during the environmental group’s demonstrations – which caused large-scale disruption in what organisers described as the biggest act of civil disobedience in recent British history – but are the first to have gone on trial with legal representation.

Patrick Thelwell, 20, from York; Peter Scott, 66, from Devon; and Samuel Elmore, 26, from Hyde End in Buckinghamshire were charged with offences including breaches of the Public Order Act 1986, obstructing a highway and obstructing police.

They were discharged by Judge Richard Blake on condition that they do not reoffend in the next year. He found them guilty after rejecting the case put forward by their barrister, Russell Fraser, that they had acted out of necessity to make the government take steps to help avert the “catastrophic consequences” from inevitable extreme global warming.

Blake urged them to learn from this experience, adding that legitimate protests do not need to break the law. But at least one, Thelwell, said afterwards that he was committed to taking part in similar actions later this year.

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The trial at City of London magistrates court heard a statementfrom McDonnell, who said the mass protests in central London led directly to MPs debating and declaring a formal climate and environment emergency.

The shadow chancellor said he and others had been inspired by the action taken by Extinction Rebellion in April, when sections of central London were shut down for days, and that the Labour party’s policy programme had developed quickly and substantially as a result.

“The activists successfully raised the profile of the climate threat and focused the minds of us all on the radical action that is needed,” he said in a statement read out in the court.

The court heard from each of the defendants, starting with Scott, a community choir leader and care home entertainer, who told the court he had started to become aware of climate change when Margaret Thatcher made a speech about it in the 1990s.

He was initially excited about the progress made through international treaties, but quickly became disillusioned and came to the firm belief that swift action needed to be taken to stave off the danger to his three children and the next generation.

“There has also been bread basket failures. Failures of fundamental crops,” he said of recent events. Speaking of how global temperatures have increased on pre-industrial levels, he said: “We are about 1.1C. When we get to 1.5C and then 2C, [crop failures] will be much worse. We are talking food shortages and all the worst. Starvation at some point. At some point mass death.

“Without disruption I am afraid nobody listens. We can stand with our placards on the side of Waterloo Bridge and watch all the cars go by and watch everybody ignore us. But what was happening was that there was an awful lot of people shouting ‘emergency’. We are in danger here and you have to make yourself heard,” Scott said.

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He was followed by Thelwell, a student at the University of York, who stood unsuccessfully this year as a Green party councillor. He told the court that the famines to come would be part of the feared sixth mass extinction. Governments including the UK’s were being “wilfully genocidal”, he told the court.

“The reality is that people are dying today from the effects of climate change.”

Blake said he did not doubt the defendants’ sincerity, adding that none of them were protesting “because they fancied a lark” and that their presence reflected their concerns about what they viewed as a potential climate disaster that threatened the future of mankind.

He noted evidence from the police about disruption which had an impact on half a million travellers, including those of limited means who were unable to leave on hundreds of coaches from Victoria bus station.

The case was not about freedom of protest, added the judge, who said the nature of the demonstration put enormous pressure on the Metropolitan police.

“It should be known in a time when we are fearful of knife violence and the epidemic of robberies which we hear about in the city that the effect of this protest was to remove those officers from being able to police them, which is disappointing to say the least,” he said.

Referring to McDonnell’s statement, he said: “I was surprised by that statement. I will say no more than that.”