– Extinction Rebellion, the more radical arm of the climate-change protest movement, on Monday kicked off two weeks of planned protests designed to shut down dozens of cities around the world.

Demonstrators blocked roads and bridges leading to the Palace of Westminster in central London. They staged a "die-in" in Wellington, New Zealand. They obstructed a major roundabout in Berlin, parked a pink sailing boat outside of the prime minister's office in Dublin and splattered fake blood on Wall Street's "Charging Bull" sculpture.

The group's message is that climate change is an emergency that requires drastic and immediate action. They have already seen some success.

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

But their tactics test public tolerance for social and economic disruption. Some say their specific demands are wildly unrealistic.

New Zealand Prime Minister Jacinda Ardern said on Monday people were free to protest "but in 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 early evening in London, police said they had arrested 217 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.

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

Anticipating police frustration, protester Paul Stephens, a retired detective sergeant, stood outside Metropolitan Police headquarters on 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 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.

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

Different strategy

Extinction Rebellion has grown in parallel to the climate strike protests inspired by teenage Swedish activist Greta Thunberg. But Extinction Rebellion includes not just schoolchildren — it has attracted young professionals, parents, grandmothers 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.

Extinction Rebellion's headliners 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," who 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 naked in Parliament and staged a "die-in" in the central hall 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 citizen assemblies, or 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.

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.