Why? To get arrested.
So far Extinction Rebellion has counted 222 arrests -- and thousands have declared they are willing to be arrested, or even go to prison, to demand action on climate change.
They estimate that significant numbers of people will have to get arrested and cause disruption for the government to pay attention to their demands. These include the UK government declaring a climate change emergency, reducing greenhouse gas emissions to net zero by 2025 and starting a citizen's assembly.
"People making decisions have to pay attention to a mass of people that come out on to the streets to demand action in the face of this crisis we are in," said Roman Paluch-Machnik, an Extinction Rebellion activist who has been arrested more than once.
"You put the police in a dilemma," explained Nuala Gathercole Lam, another protester. "If you have got thousands of people refusing to move, they either have to let you do it, which is hugely economically disruptive, or they have to arrest you."
According to the group, research shows that non-violent uprisings involving 3.5% of the public participating in acts of civil disobedience force a political response because they cannot be ignored.
"Every non-violent uprising since 1900, if it achieves that threshold, succeeds in its aims," said Paluch-Machnik. "One of our main principles is to get this 3.5% mobilized."
That would need about 2 million people to get involved in the UK.
Almost 10,000 people worldwide have signed up as "willing to get arrested," as of April 8, 2019, according to the group. Around 3,000 are based in the UK.
"Nothing has been achieved after 30 years of regular environmental campaign," said Paluch-Machnik. "People are so motivated by what is happening right now because there is not really another option."
"I've always been very worried about continuous news about the government not addressing the situation," said Alanna Byrne, an activist for Extinction Rebellion. "I felt very isolated in the way that I felt."
Along with 25 others, Paluch-Machnik, Lam and Byrne have quit their jobs to work for Extinction Rebellion full time.
In response to critics, who say Extinction Rebellion is causing unnecessary disruption and wasting police time, the activists say the "climate and ecological emergency" demands their actions.
"I think that any suffering that is caused as a result of a few hours sitting in a traffic jam is incomparable to what is going to happen in a few years" said Paluch-Machnik.
London's Metropolitan Police Service told CNN that it is aware of a number of Extinction Rebellion demonstrations and protests planned over the coming weeks, and that "Appropriate policing plans are in place."
It added: "We will always provide a proportionate policing plan to balance the right to a peaceful protest, while ensuring that disruption to communities is kept to a minimum."
This Monday, the group plans to shut down London in their biggest action yet. They plan to meet at five London locations and block traffic by playing music, hosting discussions and refusing to move from the street.
Demonstrators blocked Waterloo Bridge by bringing trees and solar panels.