Extinction Rebellion protesters are vowing to shut down Westminster as they demand urgent action on the climate and environmental crises.
Activists say they expect the protests to be as much as five times bigger than those held in April, which brought major disruption to London and saw more than 1,100 arrests.
Drumming, whistles and chanting rang through the streets as protesters attempted to block the routes into the centre of Government around Parliament and Whitehall.
The Metropolitan Police said that by 8am on Monday, 21 people had been arrested in connection with the protests.
Extinction Rebellion said those arrested included 81-year-old Sarah Lasenby, a Quaker and retired social worker from Oxford, who was part of efforts to block Victoria Embankment outside the Ministry of Defence.
She said: "The whole thing is so urgent that it is imperative the Government should take serious actions and put pressure on other states and global powers to radically reduce the use of fossil fuels, even if this means we need to reduce our comfort at home and so much flying."
The protests are targeting the heart of Government, as part of an "international rebellion" around the world, with plans to shut the streets into Westminster including Trafalgar Square and Lambeth Bridge.
Protests will also be held outside Government departments, calling on them to outline what their plans are to tackle the climate emergency, along with processions, marches and a sit-in at City Airport.
As protests got under way in the capital, police were seen cutting two Extinction Rebellion protesters out of a car blocking off Victoria Embankment.
More than 1,000 people attended an "opening ceremony" at Marble Arch on Sunday evening, featuring meditation and dancing as "inspiration" prior to the protests.
Groups of artists held a procession around Marble Arch as the protesters were told to "surround" the upcoming demonstrations with love.
On Saturday, the Metropolitan Police arrested seven women and three men on suspicion of conspiracy to cause public nuisance.
The arrests came after officers broke their way into the former Lambeth County Court building which the protesters had used to store supplies for a fortnight of protest.
Members of Extinction Rebellion said police confiscated six vehicles as well as gazebos, portable toilets and bean bags - and have since claimed that the arrests were pre-emptive.
A protest last week saw activists spray fake blood from an old fire engine outside the Treasury in protest at funding for fossil fuels.
Extinction Rebellion is calling on the Government to declare a climate and ecological emergency, to act immediately to halt wildlife loss and to reduce greenhouse gas emissions to net-zero by 2025.
They want to see the Government create and be led by the decisions of a Citizens' Assembly on climate and ecological justice.
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