Thousands of school and university students have gone 'on strike' from school as part of a global movement calling for action on climate change.
Organisers said they want the Government to declare a climate emergency and take decisive steps to tackle the issue, including drastic reforms to environmental policy and education.
Many of those protesting also want to see the voting age lowered to 16, arguing that they are the ones with the biggest stake in the future.
The Metropolitan Police said three people were arrested during protests, including a 17-year-old boy for a public order offence, along with a 19-year-old man and a 16-year-old girl who were each arrested for obstruction of the highway.
Among those taking to the streets on February 15 - along with hundreds of her classmates - was 11-year-old Scarlett Hughes, from Exeter.
She told ITV News she wants the Government to lower the voting age to allow her to have a say in her own future.
"We can't just leave it to the adults because clearly, they can't sort things out," she said.
Other protests were held in Cambridgeshire, Wales, Sheffield and London - as well as dozens of other areas stretching from Cornwall up to the Scottish Highlands.
Here's what some of the protesters had to say:
Banners called attention to what protesters called a "climate emergency", with many carrying slogans such as "there is no Planet B" and "this is environ-MENTAL".
Another read: "Why should I clean my room when the world is in such a mess?"
University of Sussex student Roseanne Steffen, 21, said she believes the climate should have been made the government's "number one priority" years ago.
We've had the climate science for 20 plus years and we've been in denial about it, so we need to react urgently.
In Cambridge, the demonstrators could be heard chanting: "Whose future? Our future" and "Hey, ho, fossil fuels have got to go."
It comes after weeks of action by young people around the world, with up to 70,000 schoolchildren from almost 300 towns and cities worldwide thought to be taking part in demonstrations and strikes each week.
In Belgium, thousands of students skipped school for six weeks running, demanding action from leaders.
Their latest protest, on Valentine's Day, saw teenagers take to the streets with banners calling "Time's Up" and "Act Now".
They were joined by others in Germany, who have held a number of protests in recent weeks too.
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."
But Government ministers and the National Association of Head Teachers (NAHT) urged students not to skip school.
In a statement, NAHT said it "could not condone" pupils missing lessons to take part in the protests.
And Education Secretary Damian Hinds added: "I want young people to be engaged in key issues affecting them and involving themselves in causes they care about.
"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."
His sentiments were echoed by the Prime Minister's official spokesperson, who said: "Everybody wants young people to be engaged in the issues that affect them most so that we can build a brighter future for all of us.
"But it is important to emphasise that disruption increases teacher's workloads and wastes lesson time that teachers have carefully prepared for.
"That time is crucial for young people precisely so that they can develop into the top scientists, engineers and advocates that we need to help tackle this problem."
Energy minister Claire Perry struck a more neutral tone, telling BBC Radio 4's Today programme that she would probably have joined the strikes herself if they had happened while she was at school.
"I'm incredibly proud of the young people in the UK who are highly educated about this issue and feel very strongly - quite rightly - that we do need to take action because it's their generation that will bear the consequences," she said.
"I do want to slightly caution that with the more official view that we can't put any more burdens on our superb teachers and teaching staff. I do hope that anyone missing school today does get their work and their homework done."
And Scottish First Minister Nicola Sturgeon openly supported the day of action, taking to Twitter to praise those involved.
"It's a cause for optimism, in an often dark world, that young people are taking a stand on climate change," she wrote.
A second round of strikes is due to be held on March 15.
In Belfast, mother Megan Hoyt went with her four children - Finn, Penny, Aisling and Isabelle - to the protest taking place at City Hall.
"We are here today because we think it's time for political action," she said.
"Personal responsibility can only take us so far. We've got our reusable coffee cups, and we've got our paper straws, and we are vegetarians and all those kind of things, but personal responsibility can only take us so far and now is the time for political leadership.
"People are angry and we are ready for something and it could be such a great moment for a new kind of politics. We are here to show there are people who want a change."