Climate change protesters blocked traffic across London's government district of Westminster on Monday as they launched two weeks of peaceful civil disobedience to call for urgent action to curb carbon emissions.
Police said they had arrested 135 activists from the Extinction Rebellion group by 1130 GMT.
The group expects 10,000 people will come to the capital from across Britain to join the two-week protest, which is part of a coordinated international movement. There were similar climate protests on Monday in Berlin, Vienna, Amsterdam, Madrid and other cities around the world.
Large crowds of protesters blocked some of Westminster's largest and busiest roads, bridges and squares, carrying banners with slogans such as "Climate change denies our children a future unless we act now".
Banging drums and chanting, they took over the tourist hotspot of Trafalgar Square and marched down the Mall, the broad tree-lined avenue that leads to Buckingham Palace.
Some activists glued or chained themselves to cars parked in the middle of roads or to street lamps, making it hard for police officers to detain them.
"We're here because the government is not doing enough on the climate emergency," said protester Lizzy Mansfield. "We only get one planet and so we're here to try and defend it."
Extinction Rebellion rose to prominence in April when it disrupted traffic in central London for 11 days. More than 1,000 activists were arrested, of whom 850 were prosecuted for various public disorder offences. So far, 250 have been convicted.
The Metropolitan Police has adopted more proactive tactics this time. Police chiefs said last week they would mobilise thousands of officers to handle the protests and that anyone who broke the law, even as part of non-violent civil disobedience, would be arrested.
On Saturday, officers used a battering ram to enter a building in south London where the activists had been storing materials to use during the two-week protest. Eight people were arrested during the raid.
Extinction Rebellion said the police response showed that British authorities considered the group a significant movement.
Early on Monday, a group of activists locked themselves to a mock nuclear missile outside the Ministry of Defence, calling on the government to redirect funds spent on Britain's Trident nuclear deterrent towards policies to combat climate change.
"Climate not Trident" read a banner by the fake missile.
Richard Dyer, a retired doctor from Scotland who was taking part in the street protests, said he regarded it as an extension of his medical career because climate change was the biggest threat yet to public health.
"People in the environmental movement and climate scientists have been trying to persuade the public and government to take serious action and nothing has happened," he said. "We want to use any way we can to make people and governments sit up and notice."