Tens of thousands of people in London joined a global day of protest Sunday to demand action on climate change, among them British actress Emma Thompson who said the challenge to save the planet was like tackling a Martian invasion.
An estimated 40,000 people marched through the British capital, according to the Avaaz campaign group, which said the turnout "massively exceeded our expectations". Police would not give an official figure.
The protest was one of thousands across the globe aimed at pressuring world leaders to take bold action on global warming at this week's UN climate summit in New York.
A sea of people walked along the River Thames past Trafalgar Square and the Houses of Parliament, many holding placards demanding "Let's do something about climate change" and a large number wearing fancy dress.
In the crowd was Oscar-winning actress Thompson, fresh from a trip this summer to the Arctic to see the melting ice sheets and meet with climate scientists.
"My daughter put it very well, she came with me to the Arctic. She said it's a bit like the Earth is being attacked by Martians, only we are the Martians," the "Sense and Sensibility" star told the BBC ahead of the march.
"If the Earth were being attacked by Martians, we would all get together and say, 'what is it that we are going to do in order to prevent the Netherlands from flooding, for example, or London being under water?'.
"We would all get together and we'd work out to how to deal with it. Which is exactly what we've got to do."