Activists poured hundreds of litres of fake blood into the road outside Downing Street to demand action on climate change.
Police stood by watching, without making any arrests, as protesters simultaneously tipped up buckets of the red liquid, which spread across the breadth of the road.
An estimated 400 demonstrators from the groups Extinction Rebellion and Christian Climate Action staged the protest, simulating a “sea of blood”.
They said pouring the liquid, which they dubbed “the Blood of our Children”, symbolised the “gravity” of the ecology and climate emergency.
The groups said they were prepared to risk arrest as part of a mass civil disobedience movement to force those in power to act.
So far, governments worldwide have failed to take sufficient meaningful action, they believe.
Scotland Yard said the policing was appropriate but would not comment while the event was still happening.
Protests were also held in other towns across the UK. In Colchester there was a “funeral march” and “lie-down” to mourn species extinctions.
They unfurled a banner reading: “Thank you for our suicide. Enjoy the feast while you still can.”
Police said those arrested were among a group who refused to leave.
An “International Rebellion” series of actions is planned in capital cities around the world from 15 April, to demand governments curb carbon dioxide emissions, limit global temperature rises and prevent mass species die-offs.
The London demonstrators called on ministers to pay as much attention to climate change as to Brexit.
“This is the biggest issue that any civilisation has ever faced. The idea that this is not the first priority of every single person and every single government is laughable,” said Teddy, Woolnough, 19.
“The climate crisis is killing people now. In London, thousands of people are dying as a result of air pollution,” said Amy Jarvis, 22.
“And on the other side of the world the situation is even worse. We are complicit in a system that is sending people to their death.”
Another activist, Jayne Forbes, said: “There are no words to describe the horrors we risk if we do not make governments act immediately. Every parent, every adult, everyone has a responsibility and duty to take action. That now means breaking the law.”
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