Protesters in London and other major European cities have blocked roads and set up camps with tents and banners as they called for urgent action on the climate and wildlife crises.
Extinction Rebellion demonstrators shut down roads around Parliament and Whitehall in London, at the start of what is expected two weeks of action in the UK capital.
The Metropolitan Police said that by 5.15pm local time, officers had made 217 arrests in relation to the protests.
But activists succeeded in bringing the area to a standstill with roadblocks on Westminster and Lambeth Bridges, Victoria Street, Whitehall, Horse Guards Road and the Mall.
Parliament Square was empty of traffic except for police vans, bicycles and rickshaws carrying tourists, while costumed protesters walked up Whitehall, and a hearse was used to block the road at Trafalgar Square.
Extinction Rebellion says the protests could be as much as five times bigger than those held in April, which brought major disruption to London and saw more than 1,100 arrests.
It is part of an "international rebellion" around the world, with action taking place in cities including Berlin, Madrid, Amsterdam and New York.
Hundreds of environmental activists took part in protests through Ireland's capital Dublin as part the action, with a mock funeral procession though the city and a large pink boat unveiled outside the heart of the Irish parliament in Leinster House.
In Berlin, around 1,000 people blocked the Grosser Stern, a traffic circle in the middle of the German capital's Tiergarten park dominated by the landmark Victory Column.
Demonstrators set up a tent camp outside Chancellor Angela Merkel's office to prepare for the protests, reflecting dissatisfaction with a climate policy package drawn up last month by her government.
In Amsterdam, hundreds of demonstrators blocked a major road outside the Rijksmuseum, one of the city's most popular tourist attractions, and set up tents.
The protest went ahead despite a city ban on activists gathering on the road. The protesters ignored police calls for them to move to a nearby square.
In Spain, a few dozen activists briefly chained themselves to each other and to an elevated road over a major artery in the capital Madrid, snarling traffic during the morning rush hour.
The National Police said 33 activists were taken to their premises and three were arrested for resisting orders by anti-riot officers.
A few hundred other protesters camped out in 40 tents at the gates of Spain's Ministry of Ecological Transition.
Around 1,000 protesters blocked the area around Chatelet in central Paris and vowed to stay at least the night in the makeshift camp they had pitched.
Australian Associated Press