More than 200 arrests in London and Amsterdam marked the beginning of Extinction Rebellion's two weeks of disruptive action to highlight what they describe as a climate emergency.
Demonstrators across Europe and worldwide have been demanding tough and immediate measures to address the issue.
The movement said it expected tens of thousands of people to answer its call to protest for two weeks in 60 cities worldwide, demanding world leaders take more decisive action to tackle the climate crisis.
Watch live footage from Monday's protests here:
In the UK, the movement asked people to converge in Westminster — the seat of the legislative power — to blockade roads.
A series of "mass participation direct actions" are also planned in the coming days including a three-day shutdown of London City Airport.
More than 1,000 Extinction Rebellion activists were arrested in London in April.
Police in Amsterdam also announced on Monday that they had arrested 90 protesters in the city centre for blocking a major street.
Demonstrations also took place in other major European capitals including Paris, Madrid, Rome and Berlin.
In the German capital, activists draped themselves in red robes signifying the extinction of species due to climate change. Others there built a symbolic Noah's ark to save endangered animals.
Activist and immigrant rescue ship captain Carola Rackete said the German government's answer so far had gone nowhere far enough.
"I myself am a nature conservation ecologist by profession and it is a disaster for me to see the federal government do nothing to prevent the climate crisis and to prevent the loss of biodiversity on this planet. These things bring humanity into crisis," she said.