LONDON — In February 2015, Simon Bramwell climbed into a tree near Bristol, in southwest England, to prevent workmen from cutting down a whole row of trees for a bus lane.
“I’m terrified of heights, so it was quite a prospect,” Mr. Bramwell, 47, said in a telephone interview.
Along with other activists, he made platforms to sit on, then spent days and nights in the trees. He relieved himself in a bucket. It was so cold, he added, that he often woke up with frost in his beard.
The protesters were evicted after a few weeks, and the trees were cut down. But that failure set Mr. Bramwell on a new path. “It was this hammer blow of, ‘O.K., that’s it, I’m done with this kind of organizing,’” he said. “‘There’s got to be a different way.’”
Three years later, following numerous meetings with like-minded activists and research into methods of achieving social change, Mr. Bramwell and 20 others assembled in a cafe in Bristol to discuss how to proceed.
It was, he said, the first official meeting of Extinction Rebellion, the climate activist group that in its short existence has arguably become the most prominent and radical climate movement worldwide. The approach those activists hit upon — using nonviolent mass disruption to increase awareness of climate change and force action on the issue — has catapulted the group to worldwide recognition and leadership on the issue.
On Monday, members of Extinction Rebellion shut down roads and bridges around Parliament in London and said they would occupy the sites for up to two weeks, resisting efforts by the police on Tuesday to move them to Trafalgar Square, another prominent site where they would have less effect on traffic.
Boris Johnson, Britain’s voluble prime minister, has called them “uncooperative crusties” — a synonym for hippies — and urged “importunate nose-ringed climate change protesters” to clear the roads.
It was the second time Extinction Rebellion had shut parts of London in six months. In April, the group installed a bright pink boat emblazoned with the words “Tell the Truth” in Oxford Circus, a junction at the heart of a major shopping district.
The group’s actions this week stretched outside London, too, with protests blocking roads as far afield as Germany, Australia, the Netherlands and New York.
In a major departure from past British climate movements, the group urges its members to try to get arrested so they can use the judicial system as a platform to force change. They tell the police about all actions in advance and view police officers as equal victims of the climate crisis. At least 280 activists were arrested in London on Monday, the police said, adding to more than 1,000 who were arrested during the April protests.
The group also calls for actions to focus on capital cities to maximize disruption, rather than more traditional sites of climate protest like power stations.
Roger Hallam, one of the group’s co-founders, who is widely seen as the driving force behind its tactics, recommends that activists emulate past movements like the United States civil rights movement and the Yellow Vests in France. “We need only a few hundred thousand people to actively break the law and/or support such activities to put us in the ballpark of structural change,” he wrote in a 78-page document, built on his research as a doctoral student, explaining why such protests were needed.
While the group’s actions may seem stronger and far more annoying than those of Greta Thunberg, the teenage climate activist from Sweden, and the School Strike movement, it has so far been largely tolerated in Britain.
In July, Policy Exchange, a right-leaning think tank, issued a report calling Extinction Rebellion “an extremist organization,” but acknowledging that it was enjoying a “honeymoon” period with the public, politicians and celebrities.
Extinction Rebellion has received support from high-profile figures like the band Radiohead, who donated proceeds from a set of tracks to the group, and Rowan Williams, the former archbishop of Canterbury. In May, the British Parliament declared a “climate emergency,” one of the group’s three demands.
Needless to say, XR, as its members call the group, has come a long way from tree protests.
Mr. Bramwell said its success was far from guaranteed. In its first months, there were sometimes only a handful of people at its meetings. But growth suddenly “snowballed.” That was the result of a number of factors, he said, from the group’s striking hourglass logo to simply “hitting that zeitgeist moment.”
The group’s first major event — a “declaration of rebellion” outside Parliament, attended by Ms. Thunberg — occurred weeks after the publication of a United Nations report saying that a climate crisis would arrive by 2040.
Extinction Rebellion’s blunt language and apocalyptic message were also critical, Mr. Bramwell said, because they reflected how many people feel about climate change. “Children are facing the bleakest of bloody futures,” he added.
Members regularly talk about the future in “end of days” terms. “We won’t survive long as we are,” said Dave Buchan, 37, protesting on Lambeth Bridge, London, on Monday, adding that food rationing would be needed.
It was Mr. Bramwell who suggested using the word “extinction” in the group’s name, emphasizing the grave nature of the threat as well as concern about wildlife and habitat. “It’s completely the opposite of Greenpeace,” he said.
Indeed, one of Extinction Rebellion’s first actions, last October, was to occupy Greenpeace’s offices in London. Activists entered the building, handed out flowers and read out a statement urging the older organization to support mass disruption.
“Failure to do things differently, when everything is failing, can only be described as complicity,” the statement said.
Extinction Rebellion campaigners continually debate whether actions should be undertaken that might risk alienating the public. There is “a constant waltz” between members who want more direct action and those who don’t want to annoy the public, Mr. Bramwell said.
In April, there was heated discussion over whether to target London’s public transport system. In the end, two activists stood on the roof of one train. A plan to fly drones in an exclusion zone around Heathrow Airport was also dropped after internal debate, though it was carried out in September by members using the name Heathrow Pause.
Extinction Rebellion has threatened further acts of disruption at London City Airport, a far smaller transit hub favored by business travelers.
Melanie Edwards, 48, a Welsh activist who was among those arrested in September in the Heathrow action, said that Extinction Rebellion’s tactics explained its success.
“You can have a million people marching through the city each week and no one cares,” she said, “but you block a road, people stand up and take notice.” Such activities have also inspired people to join, she added.
Perhaps the best way to see the movement’s impact is not on London’s streets or online, where it has hundreds of thousands of followers, but at the City of London Magistrates’ Court in the heart of London’s financial district.
There, every Friday for the past two months, several courtrooms have been dedicated to processing Extinction Rebellion members arrested during April’s protests for disobeying a police order to move on. Some 279 cases had been concluded by Oct. 4, Britain’s Crown Prosecution Service said in an email, with 478 yet to be finished, 295 of those adjourned for trial.
One day recently, a young man wearing a suit — his only one, bought while at university, he said — waited nervously outside a courtroom along with about a dozen other activists, He did not want to be interviewed, he said, because he was worried his employer would find out.
Inside, his hands shook as he pleaded guilty then read out a statement to the judge and a handful of supporters. He had become aware of the climate crisis during school geography lessons, he said.
Shortly afterward, Vanessa Stevenson, 58, a gardener who had traveled several hours south from Derbyshire for the hearing, pleaded guilty too. The police had found her “lying on an inflatable mattress under an Extinction Rebellion banner” in Parliament Square, the prosecutor said. She had refused to leave.
Ms. Stevenson read out her own statement, saying among other things that her daughter had promised not to have children because of climate change. “We are supposed to make the world a better place for our children, not use up their future,” she said, starting to cry.
After Ms. Stevenson finished her speech, the judge thanked her and the other two defendants. “I understand why you did it,” he added, but then pointed out they had still broken the law and fined them each 85 pounds, plus a victim surcharge, a total of about $130.
Outside, Ms. Stevenson said the fine would not deter her from joining protests. “I have to,” she said. “Until things change, I’ve got to keep doing this.”