Schoolchildren are taking a stand against climate change, with thousands of pupils walking out of class to demand immediate action.
Youth Strike 4 Climate organisers said strikes are taking place in 60 towns and cities, with youngsters carrying banners bearing slogans such as "There is no planet B".
Other signs read "When did the children become the adults?" and "Why should I clean my room when the world is in such a mess?".
At one protest outside Cambridgeshire County Council's offices, a demonstrator led chants of "Whose future? Our future" and "Hey, ho, fossil fuels have got to go".
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Jasper Giles, a six-year-old pupil at University of Cambridge Primary School, attended with his mother Alissia Roberts.
She said: "I think it's worth taking a day off school to show support for this movement. I think it's really important and it will gather momentum."
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Ten-year-old Zachary Hird, a pupil at Cambridge's Newnham Croft Primary School, was at the protest with his mother Diane Hird.
He said: "We don't want climate change and people just have to change their ways, as we don't want the world as it is right now.
"We just want to make people aware of it. We were talking about it in our class, so we just came along."
Asked how he felt about missing lessons for the day, he said: "I feel climate change is more important – the world dying is a lot more bad."
Children also gathered in Brighton, with some of those waving banners refusing to take exams and calling for immediate action on climate change.
The protesters also took the cause to Westminster, descending on Parliament Square to demand change.
Dressed in their school uniforms, some broke into chants of "Save our planet" and "Now, climate justice".
Former UN climate chief Christiana Figueres voiced her support for the cause, saying the action was "moving".
She said: "It's time to heed the deeply moving voice of youth and schoolchildren, who are so worried about their future that they need to strike to make us pay attention.
"It is a sign that we are failing in our responsibility to protect them from the worsening impacts of climate change."
However, the strikes were not welcomed by school leaders and education secretary Damian Hinds, who said missing class was not the answer.
"I want young people to be engaged in key issues affecting them and involving themselves in causes they care about," he said. "But let me be clear, missing class won't do a thing to help the environment; all they will do is create extra work for teachers."
However, the demonstrators gained the support of Scotland's first minister, Nicola Sturgeon, who described their actions as a "cause for optimism in an often dark world".
The movement has already seen school strikes in Australia and European countries including Belgium, and was inspired by teenager Greta Thunberg, who protests every Friday outside Sweden's parliament to urge leaders to tackle climate change.
Students in the UK are demanding that the government declares a climate emergency and takes active steps to tackle the problem, communicate the severity of the ecological crisis to the public and reform the curriculum to make it an educational priority.
Anna Taylor, of UK Student Climate Network, said: "We're running out of time for meaningful change, and that's why we're seeing young people around the world rising up to hold their governments to account on their dismal climate records.
"Unless we take positive action, the future's looking bleak for those of us that have grown up in an era defined by climate change."