The group’s message is that climate change is an emergency that requires drastic and immediate action.

And it has already seen some success.

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“Extinction Rebellion is widely credited with accelerating policy change in the U.K.,” said Robert Falkner, a fellow at Chatham House, a think tank.

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But the group’s tactics test public tolerance for social and economic disruption. And some say Extinction Rebellion’s specific demands are wildly unrealistic.

New Zealand Prime Minister Jacinda Ardern said on Monday that people were free to protest but that “blocking people from being able to go and do their day-to-day job doesn’t necessarily take us any closer to the climate action they are calling for.”

By evening in London, police said they had arrested 276 demonstrators. The last time Extinction Rebellion staged a protest on this scale, in April, more than 1,000 people were arrested, in a police operation that cost nearly $20 million.

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London Mayor Sadiq Khan conceded that “bolder action” is needed to take on climate change, but he criticized the movement for potentially overwhelming an already stretched police force.

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Anticipating police frustration, protester Paul Stephens, a 55-year-old retired detective sergeant, stood outside Metropolitan Police headquarters Monday morning handing out fliers to officers.

“From their perspective, it’s a waste of time; from our perspective, it’s not,” he said, explaining that the strategy was to “create a dilemma for the police” so they either had to allow the demonstrations to continue or arrest 1,000 “otherwise law-abiding people” for committing low-level offenses.

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The protesters are walking a tightrope — they want to spark enough disruption to effect change but not so much that they alienate the public.

Extinction Rebellion has grown in parallel to the climate strike protests inspired by teenage Swedish activist Greta Thunberg. It has attracted not just schoolchildren but also young professionals, parents, grannies and others concerned about climate change and habitat loss. And, pretty much from the beginning, Extinction Rebellion’s strategy has been very different from that of the students skipping school on Fridays to protest peacefully.

The group was launched a year ago, in a small English town in the Cotswolds, by activists from an organization called Rising Up. Their first large demonstration took place last November, when activists blocked five bridges in London and police made 85 arrests.

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Extinction Rebellion’s founders include Gail Bradbrook, a former biophysicist, and Roger Hallam, who is in prison for his role in “Heathrow Pause,” a splinter group that planned to shut down Heathrow Airport with low-flying drones.

But organizers say Extinction Rebellion is not reliant on leaders. On the streets in London, protesters can join small “affinity groups,” which support each other and communicate largely via encrypted mobile apps Signal and Telegram. Some crowdfund legal fees for those who want to be an ­“arrestable.”

To get attention for their cause, protesters have glued themselves to roads, stripped in Parliament and staged a “die-in” at London’s Natural History Museum.

Within Britain, Extinction Rebellion has three demands of government: to “tell the truth” by declaring a climate emergency; to pledge to reach net-zero greenhouse-gas emissions by 2025 and halt the loss of biodiversity; and to enlist a citizen assembly — a group of representatives from the wider public — to guide the way forward.

Along with the student climate strikers and British naturalist David Attenborough, they have helped to change the content and tone of the conversation about climate change.

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A day after Extinction Rebellion representatives met with government officials, the British Parliament declared a climate emergency. A few weeks later, the government pledged to be carbon neutral by 2050, the first major economy in the world to do so.

“It’s no longer a question of whether we should act. It’s what’s reasonable action to take and how fast can we go,” said Bob Ward, a spokesman for the London School of Economics’ Grantham Research Institute on Climate Change and the Environment.

Britain’s 2050 target is much further out than what Extinction Rebellion wants. But Ward said the government’s goal is already ambitious and will require massive changes to the transport sector and how people heat their homes. Extinction Rebellion, he said, is not “offering an alternative analysis that shows 2025 is doable.”

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Organizers are aware that their demands are unlikely to be met as the result of the next two weeks of protests. They are unsure of what comes next. “We don’t want this to become like Glastonbury, something fun to do every April and October,” said one organizer, speaking on the condition of anonymity to discuss internal deliberations.

Clare Farrell, one of Extinction Rebellion’s co-founders, said there has been tension inside the group over tactics. The Washington Post spoke with her at the group’s office in east London, where the walls are plastered with posters that read “Rebel for Life” and others bearing the group’s symbol, a circle to represent the planet and an hourglass to show that time is running out.

Farrell recalled an April demonstration in which activists glued themselves to a train in London’s financial district.

It was not an easy sell inside the group. “Some still thought it was radical, some are just getting to terms with the fact they can sit on a road. . . . We need to make sure we don’t push too hard, too fast, in one place.”

This month’s protests are the group’s boldest — and most expensive — yet.

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Andrew Medhurst, head of finance, said the group earmarked about $1.2 million for October in London to pay for entertainment, portable toilets and meals for about 20,000 people a day. The group also pays about 190 of its activists, who can claim up to $500 a week.

Over the past year, Extinction Rebellion has raised $2.8 million through a mix of crowdfunding and big donations from sources including the British rock band Radiohead and the U.S.-based Climate Emergency Fund, whose founders include philanthropist Aileen Getty and filmmaker Rory Kennedy.

Medhurst worked for three decades in London’s financial district before quitting his job.

“I suddenly got a big kick up my backside,” he said. “I realized that unless there were drastic changes, my kids won’t need a stock portfolio, they will need food and shelter.”

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Many activists in the streets of London on Monday echoed that idea.

They included Vishal Chauhan, 30, who gave up his job as an emergency room doctor, where he earned around $80,000 a year, to devote himself to Extinction Rebellion.

“I love being a doctor,” Chauhan said, “but it doesn’t make sense being in hospital treating people for diabetes or pneumonia knowing that they or their children might experience TB, cholera, famine, food shortages.”

He said part of him hopes a warming world is not as catastrophic as scientists predict.