Police have started arresting climate change protesters as they attempt to block roads in central London.
Thousands of people are expected to demonstrate over the next two weeks and witnesses say that their numbers are already overwhelming police in some streets.
Two people have blocked the road outside New Scotland Yard after chaining themselves to a metal cylinder inside a car.
Police are using cutting equipment to free them.
Extinction Rebellion, which has organised the protests, told Sky News’ Enda Brady that 23 people have already been arrested, including 81-year-old Sarah Lasenby.
The quaker and retired social worker, from Oxford, was arrested alongside others from the XR Peace group who were blocking Victoria Embankment outside the Ministry of Defence.
Ms Lasenby said: “For 21 years my main concern has been to help get rid of UK nuclear weapons.
“I am still keen to do this but once I came across XR I was so relieved to have something I could do about the ghastly state we have got our planet in.”
She added: “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.”
Protesters have blocked both ends of Lambeth Bridge in central London, one of 12 sites where demonstrations are taking place.
Sky’s climate change correspondent Hannah Thomas-Peter has said it appears around 300 people are on the bridge.
She spoke to an elderly protester called Diana who said she was willing to be arrested today.
The protester told Sky News: “Unless we do something dramatic and make a lot of trouble, this terrible climate change disaster is going to continue happening.
“We need to raise the profile, we need the government to listen, and whatever age, whatever sex agenda, we need to be out here.
“I’ve decided I’ve got nothing to lose.”
A 25-year-old teacher from London who took the week off to join the protests said: “The plan is to shut down the whole of London.
“The issue of climate change is an issue for everyone, and it’s clear we can’t leave it to the politicians.
“I booked the week off work for this.
“The school kids are out protesting too. Everyone is.”
Police are searching anyone suspected of carrying paint.
Activists are not only targeting London.
Major cities around the world are being targeted, with Amsterdam already seeing one major road being blocked.