Police have so far arrested 21 climate activists, they said on Monday, at the start of two weeks of planned peaceful civil disobedience in London that will call for urgent government action to curb carbon emissions.
The protests, which organizers expect to bring 10,000 people to the British capital, are part of what Extinction Rebellion activists call an "international rebellion", with similar actions taking part in Australia, the Netherlands and elsewhere.
Reuters reporters in London's government district of Westminster saw activists block a bridge and several roads as the protests gathered pace early on Monday.
A small group of activists locked themselves to a mock nuclear missile outside the Ministry of Defence, calling on the government to redirect the funds spent on Britain's Trident nuclear submarines towards climate policy.
"Climate not Trident" read a banner deployed next to the fake missile.
At another location, a pair of activists parked a car in the middle of a major road and attached themselves to it. A Reuters photographer saw more than a dozen police officers surrounding the vehicle as two of them tried to cut the activists free.
Separately, large groups of activists blocked Lambeth Bridge and Parliament Street, both locations close to the Houses of Parliament. They carried banners with slogans such as "Climate change denies our children a future unless we act now".
Richard Dyer, a retired doctor from Scotland who was taking part in the 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 told Reuters. "We want to use any way we can to make people and governments sit up and notice."
Extinction Rebellion came to prominence in April when it disrupted traffic in central London for 11 days. More than 1,000 activists were arrested at that time, of whom 850 were prosecuted for various public disorder offences. So far, 250 have been convicted.
The police have adopted more proactive tactics to deal with the group this time.
On Saturday, officers used a battering ram to gain entry to a building in south London where the activists had been storing equipment for use during the two-week protest.
The police arrested 10 people during the raid, on suspicion of conspiracy to cause public nuisance. By 8 a.m. (0700 GMT) on Monday the number of arrests had risen to 21 people.
Extinction Rebellion said the arrests showed that British authorities considered the group a significant movement.
The Metropolitan Police said last week they would mobilise thousands of officers to handle the planned protests and that anyone who broke the law, even as part of non-violent civil disobedience, would be arrested.