Video report by ITV Science Editor Tom Clarke
Protesters have begun what they say will be two weeks of disruption - targeting 11 sites across the capital - amid planned action in 60 cities worldwide.
While several hundred were held on Monday, many ended the night in tents outside the Houses of Parliament.
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The day started with protesters blocking traffic from entering the roundabout at Trafalgar Square and two activists mounted a Land Rover and trailer.
Other protesters lay under the trailer as police surrounded the vehicles.
A funeral car was later parked in Trafalgar Square, containing a coffin which read Our Future and its driver was locked to the steering wheel, while other protesters attached themselves to the hearse.
The roads behind Downing Street were blocked throughout the day by two groups of protesters, some of whom had erected tents in the street and were sat down singing songs together.
Video report by ITV News Correspondent Rachel Younger
What does the group have planned in Westminster?
Protests are being held outside Government departments across Westminster, with protesters calling on them to outline what their plans are to tackle the climate emergency.
Processions, marches and a sit-in at City Airport are also planned, while a group have started a yoga session on Westminster bridge.
The group says they have already amassed at Trafalgar Square, Westminster Bridge, Lambeth Bridge Millbank, Victoria, The Mall, Whitehall, Victoria Embankment, Horse Guards Parade and Victoria Street.
Elsewhere, hundreds of activists staged a sit in at the famous Smithfield meat market in protest at the carbon footprint of the meat industry.
Drumming, whistles and chanting rang through the streets as protesters attempted to block the routes into the centre of Government around Parliament and Whitehall.
Lambeth and Westminster Bridges have been shut by protesters and one protester chained themselves to a missile outside the Ministry of Defence.
ITV News Correspondent Rachel Younger has been looking at the impact of the protests.
Who are the people at the protest?
Actress Juliet Stevenson was one of several celebrities taking part in the protests at Trafalgar Square, along with actress Ruby Wax and models Daisy Lowe and Arizona Muse.
The Truly Madly Deeply star said: "It's a very wonderful action today. We can't any longer allow governments to do this so we have to make it clear that there is no more time.
"There's a long tradition in this country of people saying governments are not acting, we have to make them realise how urgent this is.
"I'm optimistic about the energy there is amongst people to act but I'm not hugely optimistic about government stepping up to the plate.
"They're [the government] talking about 2050 and scientists have said we have 12 years before we're in a place where the climate is irreversibly damaging our planet and we won't be able to repair or fix it. We need to make them realise that time is not on our side at the moment."
Ms Stevenson said she was delighted to see so much engagement from young people and that her own son was at the protests and working for Extinction Rebellion.
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.”
Writer and environmental activist George Monbiot is quoted by the group as saying: “This is the most important mobilisation of my lifetime.
"We are facing the greatest predicament humankind has ever faced, and we need to meet it with the biggest movement there has ever been.
"It is brilliant to see so many people with different skills and perspectives coming together to build this mass mobilisation.”
19-year-old Lili Rowe, from Leeds, is protesting in London and claims her outfit is made from rubbish she collected in 30 minutes.
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.
Where else in the world are protesters out in the streets?
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.
Activists have also blocked major roads in Berlin and Amsterdam while protests continue in major cities across Europe, including Paris, Vienna, Rome, Prague, Bratislava and Budapest.
Further afield, members are demonstrating in various Indian cities, Tokyo, Istanbul, Hong Kong, Cape Town, Toronto and New York.
Around 1,000 people blocked the Grosser Stern, a traffic circle in the middle of the German capital’s Tiergarten park dominated by the landmark Victory Column in a protest that started in the early hours.
Members of Extinction Rebellion have also set up a camp outside German chancellor Angela Merkel’s office, reflecting dissatisfaction with a climate policy package drawn up last month by her government, ahead of what it called an “international rebellion”.
The protest group says demonstrations are planned in 60 cities worldwide.
In Amsterdam, hundreds of demonstrators blocked a major road outside the Rijksmuseum, one of the city’s most popular tourist areas, and set up tents.
The demonstration went ahead despite the city banning activists from gathering on the road.
The protesters ignored police calls for them to move to a nearby square.
Protester Elle van Zeeland told Dutch broadcaster NOS that the group is “staying here until the government meets its obligations”.
Founded in Britain last year, the movement, also known as XR, now has chapters in some 50 countries.
Mrs Merkel’s chief of staff, Helge Braun, criticised the group’s tactics: “We all share an interest in climate protection, and the Paris climate targets are our standard in this,” he told ZDF television.
“If you demonstrate against or for that, that is OK, but if you announce dangerous interventions in road traffic or things like this, of course that is just not on.”
He dismissed the idea of declaring a “climate emergency”, saying that the constitution does not provide for such a thing and it does not translate into “concrete action”.
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.