Protesters in London are trying to block five of the capital's bridges to demand urgent action on climate change.
The group Extinction Rebellion is staging a sit-in on Blackfriars, Waterloo, Westminster, Lambeth and Southwark bridges.
Coordinated action is taking place at sites around the UK and the globally.
The largest crowds were on Westminster Bridge, with police keeping watch as protesters – some carrying banners – blocked the normally busy bridge.
Sky News reporter Phil Edwards said there were around 1,000 people taking part in the protest next to the Houses of Parliament.
Extinction Rebellion claims scientific data shows the enormity of climate change has been ignored by governments and that ecological catastrophe, and social collapse, is possible within a generation.
One of the protest organisers, Dr Gail Bradbrook, said: "The UN has given us until 2020 to change the course of humanity.
"If that doesn't happen human extinction in my children's lifetime is a possibility."
"People need to take their heads out the sand and look at the science and frankly panic and join us. It's extremely frightening."
3:02Dr Bradbook said blocking the bridges was necessary after other protests, marches and letters to MPs had achieved little.
"Nothing is being done of any real significance," she said.
"Our demand is the government must tell the truth about the crisis we're in. And that includes working with communities to build resilience… We want to go to net carbon zero emissions by 2015 and reduce our consumption levels."
Two women who did not want to be named were at the protest dressed as bumble bees.
"I have concern for my children and grandchildren. I truthfully believe we're all heading for extinction," one said.
"Climate change is so important, it's coming over so fast and nothing is being done."
Other people at the demonstration were there to urge the government to stop the controversial process of "fracking" for gas – which recently resumed in the UK.
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Organisers estimate that 50 people have been arrested for taking part in action over the last week.
Twenty-two people were arrested on Monday, when protesters glued themselves to gates at the Department for Business, Energy and Industrial Strategy.
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