Campaign group Extinction Rebellion is targeting five central London locations.
Environmental protesters have blocked some of London’s busiest roads in a bid to bring the capital to a standstill.
Activists are targeting five central locations as they demand the Government declare a climate emergency.
The first human roadblock closed Waterloo Bridge to traffic in both directions on Monday morning before protesters descended onto the roads at Oxford Circus and Marble Arch.
Hundreds of people gathered outside Oxford Circus Tube station around a bright pink boat, where some used makeshift devices to lock their arms together.
In Parliament Square, others unfurled banners, held up placards and waved flags as speakers took to the stage.
Campaign group Extinction Rebellion is expecting thousands to join the peaceful protests, which could last for weeks, as they demand the Government takes urgent action on climate change and wildlife declines.
Organisers have warned activists they could face arrest for blocking traffic and Transport for London said travellers face disruption and delays.
The movement has received support from actress and activist Dame Emma Thompson and former archbishop of Canterbury Dr Rowan Williams.
Speaking at a meditation on the eve of the protests Dr Williams said humans had declared war on nature.
He said: “We are here tonight to declare that we do not wish to be at war.
“We wish to make peace with ourselves by making peace with our neighbour Earth and with our God.”
Thompson has previously said of the demonstrations: “It is time to stand up and save our home.”
Organisers said: “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.”
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 were warned they could be breaking the law by doing so is 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 is not allowed.
But police said no arrests had been made by midday on Monday after earlier explaining 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.
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