Extinction Rebellion protesters have brought Westminster to a halt as they demand the Government takes urgent action on climate change and wildlife losses.
Groups of demonstrators shut down roads around Parliament and Whitehall, with vehicles and bikes, and banners reading "tell the truth" and "No coal mines, no fracking".
Parliament Square was empty of traffic except for police vans and bicycles, while helicopters circled overhead, as protesters created roadblocks on Westminster and Lambeth Bridges, Victoria Street, Whitehall and the Mall.
Some 21 people had already been arrested by 8am, the Metropolitan Police said, as what is expected to be two weeks of action in the capital got under way.
Activists say the protests could 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.
It is part of an "international rebellion" around the world, with action taking place in cities including Berlin, Madrid, Amsterdam and New York.
In the UK, 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 also want to see the Government create and be led by the decisions of a Citizens' Assembly on climate and ecological justice.
Extinction Rebellion protester Caroline Hartnell, 69, from London, said activists were going to be surrounding all the Government ministries.
"We are going to be putting pressure on them - what they are going to do mend the climate emergency, because we are running out of time.
"I have seven grandchildren and the youngest is three. I feel passionately for them (and worry) there is not going to be a world for them to live in."
Oxford University student Fergus Green, from St Albans, was in the growing crowd of drummers, stewards and flag-waving protesters planning to "shut down Westminster".
He said: "People are coming from all over the country. I'm here to force the Government to take action on climate change. I'm a student, I should be at uni - my term has started.
"But I'm here to take action. People are missing work and school to be here."
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