Extinction Rebellion protesters have blocked off central London as part of a global demonstration.
They are calling for the Government to declare a climate emergency.
Oxford Circus, one of the city's busiest shopping and tourist districts, is blocked by a huge pink boat being paraded through the street.
This morning, commuters also faced traffic chaos in the city centre as they targeted areas including Parliament Square, Waterloo Bridge and Marble Arch.
The group brought shrub trees to Waterloo Bridge, blocking traffic in both directions.
They also placed flowers along lane markings while calling out for change and banging drums.
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Last week, members of the activist group glued themselves naked to the glass partition of the House of Commons public gallery.
The movement, which is demanding the Government takes urgent action on climate change and wildlife declines, has received support from actress and activist Dame Emma Thompson and former archbishop of Canterbury Dr Rowan Williams.
The pink shop driven through London's Oxford Circus was branded the 'ship of truth', attracting crowds to block off the area's major junction.
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London spokesperson for the group said: "The governments of the world have failed us and continue to fail us.
"They seem utterly divorced from the reality which the scientists warn is bearing down upon us and preoccupied with furthering the same aims and pursuits which have brought us to this head.
"If we are to save ourselves, then it is up to ordinary people to force them to change their ways. This our last best shot at survival."
Organisers added in a statement: "The International Rebellion begins and Extinction Rebellion will be bringing London to a standstill for up to two weeks.
"They will be blocking five of the city's busiest and most iconic locations in a non-violent, peaceful act of rebellion where they invite people to join them for several days of creative, artist-led resistance."
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Demonstrators arrived at London's Hyde Park on Sunday, some having journeyed to the city on foot in recent weeks from various parts of the UK for what is described as an "International Rebellion".
Monday will see people in at least 80 cities in more than 33 countries hold similar demonstrations on the same environmental issues, campaigners said.
While organisers encouraged people to set up camp in Hyde Park overnight into Monday, they have been warned they could be breaking the law by doing so is an offence under Royal Parks legislation.
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A spokeswoman for The Royal Parks said Extinction Rebellion had not asked for permission to begin the protest in the park and that camping is not allowed.
Police said their operational response to camping "would be dependent on what if any other issues might be ongoing at the time".
Scotland Yard said they have "appropriate policing plans" in place for the demonstrations and that officers will be used from across the force "to support the public order operation during the coming weeks".
Police advised people travelling around London in the coming days to allow extra time for their journey in the event of road closures and general disruption.
