More than 130 people have been arrested in London at the start of two weeks of protests by environmental campaigners.
Extinction Rebellion activists are protesting in cities around the world, including Berlin, Amsterdam and Sydney.
Organisers have planned to shut down key sites in central London, in addition to demonstrating outside government departments.
Extinction Rebellion claims protests in the capital will be five times bigger than similar events in April.
The protests are calling for urgent action on global climate and wildlife emergencies.
Activists barricaded themselves to vehicles in Westminster early on Monday as the demonstrations got under way.
Police were seen cutting two protesters from a car that had blocked Victoria Embankment, while campaigners also locked themselves to a mock Trident missile outside the Ministry of Defence in Whitehall.
Activists were also pictured on a barge on the Thames, according to BBC reporter, Bruce Thain.
Meanwhile, hundreds of campaigners have filled Trafalgar Square and blocked Lambeth and Westminster bridges.
A string of celebrities including fashion model Daisy Lowe, actress Juliet Stevenson and comedian Ruby Wax, joined campaigners at Trafalgar Square.
Ms Stevenson said the protests were “a very wonderful action”, revealing her son was attending them as a worker for Extinction Rebellion.
She told the Press Association (PA): “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… We need to make them realise that time is not on our side at the moment.”
On Saturday Ms Lowe, 30, hosted a dinner to “celebrate and be educated” by Extinction Rebellion activists, and encouraged followers to join the protests.
She wrote on Instagram: “It is a terrifying reality we live in, but we have the power to change the course of history and save our planet.”
Sir Mark Rylance, the Oscar-winning actor joined a blockade in the Mall, and said “urgent action” was needed, The Times reports.
He told the newspaper: “We want a much deeper discussion and more urgent action.
“I’m a storyteller and this story dwarfs everything else.”
He added that he would not rule out being arrested, saying it was “worthwhile” for certain things.
“That depends on what situation arose. XR is here because the democratic process has failed.”
“I do think there are things that it’s worthwhile being arrested for and you can look back at many of the things that have changed in the past, people have been arrested.
“Notorious people like me who write letters and be arrested that’s not it, it’s many little changes in people’s hearts.”
In June, Sir Mark resigned as an associate artist at the Royal Shakespeare Company (RSC) over its partnership with BP, which the theatre company has since vowed to end.
Meanwhile, activists from Animal Rebellion, a movement allied to Extinction Rebellion, began marching from Russell Square to Smithfield Meat Market on Monday afternoon.
Organisers said they plan to stage an overnight occupation of the market to share their “vision of a future plant-based food system”.
Fiona Oakes, the world record holding England long-distance runner and vegan campaigner, joined the action.
In an update shortly before 13:00 BST, organisers said several thousand people had blocked locations across Westminster, including Whitehall and the Mall.
A spokesperson for the Metropolitan Police said there were 135 arrests in connection with the protests as of 12:30 BST.
Extinction Rebellion said this included Sarah Lasenby, 81, a Quaker and retired social worker from Oxford.
Ms Lasenby, who the group says was part of efforts to block Embankment, said: “It is imperative that the government should take serious actions and put pressure on other states and global powers to radically reduce the use of fossil fuels.”
What is Extinction Rebellion?
2025group’s aims for zero carbon emissions
298,000followers on Facebook
1,130people arrested over April’s London protests
2018year the group was founded
Source: BBC Research
Extinction Rebellion (XR for short) wants governments to declare a “climate and ecological emergency” and take immediate action to address climate change.
It describes itself as an international “non-violent civil disobedience activist movement”.
Extinction Rebellion was launched in 2018 and organisers say it now has groups willing to take action in dozens of countries.
In April, the group held a large demonstration in London that brought major routes in the city to a standstill.
Extinction Rebellion organisers say they are expecting up to 30,000 people to take part in the fortnight-long demonstrations in the capital, which form part of an “international rebellion”.
Similar protests in the UK earlier this year brought major disruption to London and resulted in more than 1,100 arrests.
Up to 60 other cities around the world may also be disrupted in simultaneous events, according to a spokesperson for the group.
Activists will call on government departments to detail their plans to tackle the climate emergency.
Police in Australia and New Zealand have already arrested dozens of Extinction Rebellion activists on Monday.
Some 30 campaigners in Sydney were charged with committing offences after hundreds of protesters blocked a busy road.
The latest arrests in London come after the Met police arrested 11 people during the weekend.
A spokesperson for the force said eight people were arrested on Saturday after previously reporting 10. They have all been released under investigation.
One woman and two men were arrested on Sunday on suspicion of conspiracy to cause public nuisance. The men remain in custody while the woman has been released under investigation.
More than 1,000 people attended an “opening ceremony” at Marble Arch on Sunday night, which featured meditation and dancing.