Climate change protesters have blocked roads across central London sparking traffic disruption.
Members of campaign group Extinction Rebellion have also parked a boat at Oxford Circus, and blocked Marble Arch, as part of a global day of action.
Activists smashed the glass of the revolving doors of oil company Shell’s London headquarters in Waterloo.
Police have advised people travelling into London to allow extra journey time.
Yen Chit Chong, from Extinction Rebellion in London, said: “This is our last best shot at survival.”
Organisers claim protests are being held in over 80 cities across 33 countries.
Extinction Rebellion said protests would continue throughout the week “escalating the creative disruption across the capital day by day”.
The group said it planned to “bring London to a standstill for up to two weeks”, and wanted the government to take urgent action to tackle climate change.
In Parliament Square, protesters unfurled banners, held up placards and waved flags as speakers took to the stage.
Who are Extinction Rebellion?
Since its launch last year, members have shut bridges, poured buckets of fake blood outside Downing Street, blockaded the BBC and stripped semi-naked in Parliament.
It has three core demands: for the government to “tell the truth about climate change”, reduce carbon emissions to zero by 2025, and create a citizens’ assembly to oversee progress.
Controversially, the group is trying to get as many people arrested as possible.
One of the group’s founders, Roger Hallam, believes that mass participation and civil disobedience maximise the chances of social change.
But critics say they cause unnecessary disruption and waste police time when forces are already overstretched.
By intentionally causing more than £6,000 damage at the Shell headquarters activists aim to get the case into crown court to put their case to a jury, the campaign said.
Protester Chay Harwood told the BBC: “We live in a very sick society at the moment. There’s a lot of social issues and social ills that need curing.
“But at the moment the biggest threat we face is the threat of climate change.”
The Met said it had “appropriate policing plans” in place for the demonstrations and officers from across the force would be used “to support the public order operation”.
In November, activists blockaded the Department for Business, Energy & Industrial Strategy by chaining themselves together on the pavement, leading to 85 arrests.
At the scene
By Matthew Cannon, BBC News
The unusual site of a pink yacht stands in the centre of Oxford Circus, surrounded by a sea of coloured flags.
One man who plans to protest here through the night says the protests will be peaceful but he is willing to be arrested.
“The more the authorities will get fed up with us the more it brings us to their attention,” he said.
Organisers have encouraged people to set up camp in Hyde Park overnight into Monday – an offence under Royal Parks legislation.
A spokeswoman for The Royal Parks said Extinction Rebellion had not asked for permission to begin the protest in the park and that camping was not allowed.