Video report by ITV News Correspondent Dan Rivers
Millions of people have taken to the streets all over the world in what was the largest climate protest in history.
From Australia to Europe and on to America, the protesters called on businesses and politicians to cut emissions and act on the climate crisis.
In Britain, children and young people are walked out of schools and lectures, with hundreds of thousands of adults joining them for the first time.
According to organisers, 100,000 people have taken part in a rally in central London, with more than 20,000 thought to have marched in Edinburgh and 10,000 in Brighton as crowds flocked on to the streets around the UK.
In Belfast, the turnout was estimated to between 3,000 and 4,000, with young people taking over the Corn Market area of the city centre and staging a "mass die-in", before marching towards City Hall.
Thousands of protesters, including hundreds of children, many wearing school uniform, gathered in Birmingham's Victoria Square before marching through nearby streets.
ITV News Science Editor Tom Clarke discusses how these protests will galvanise political action on climate change from world leaders
Some ministers and school leaders spoke out against pupils missing school to take part in the protests, but London Mayor Sadiq Khan, Green Party MP Caroline Lucas and Liberal Democrat leader Jo Swinson were among those backing the walkout for the Youth Strike 4 Climate campaign.
Mayor of London Sadiq Khan said: “It is unbelievable that we should need global strike action for the future of our planet to be taken seriously.
“The stark reality is that our climate is changing rapidly and we are running out of time to address it.
"I hope governments around the world who are failing to take action hear the voices of millions of people, young and old, unified in their call for action to save our planet. Our future depends on it."
Labour leader Jeremy Corbyn addressed a rally of climate strikers outside Parliament on Friday, while other events were being held up and down the country.
He said: "We all suffer as that carbon sink is destroyed, destroyed in order for somebody to make a short term profit.
"We are destroying species we don't even know of their existence yet and so all of us should put all of our demands on our placards."
While the protests were mainly peaceful, Met police made seven arrests for public order offences for breaching conditions set on the protest - in that protesters must gather in a specific place in Westminster.
One man was arrested under the Explosives Act for releasing a flare on Lambeth Bridge.
The first of Friday's protests kicked off in Australia, where an estimated 300,000 people gathered at more than 100 rallies calling for action to guard against climate change.
Demonstrations were also held across Europe, Asia, Africa and the US.
The protests are part of a snowballing movement sparked by teenage activist Greta Thunberg’s school strikes outside the Swedish parliament.
These monumental protests come ahead of a climate action summit in New York convened by UN secretary general Antonio Guterres, urging countries to up their climate efforts.
Much steeper measures are needed across the globe to prevent temperature rises of more than 1.5C or 2C to avoid the worst impacts of climate change.
As if to underline the urgency of the issues, the mercury is set to hit 26C this weekend - 8C above average for the time of year.
Children across the UK have missed lessons for what they say is the most important lesson of all, to tackle climate change. and to save a future they might not have if the planet continues down the current path.
Aerial footage of the climate strike in Brighton
The UK Student Climate Network (UKSCN) said more than 200 events took place across the UK, with adults being encouraged to join the youngsters with the strikes for the first time.
UKSCN is calling on politicians to bring in a “Green New Deal” to cut the UK’s emissions to zero and improve lives, changes to education to equip youngsters to deal with the climate crisis and votes at 16 to give them a voice.
Among the many trade unions who threw their weight behind the strikes are the TUC Congress, the University and College Union and Unite, while environmental campaigners, aid agencies and faith groups joined the protests.
Some companies were actively helping staff take part by closing stores and offices, including Patagonia and Ben & Jerry's, whose sign on its central London shop on Wardour Street read: "Gone striking. Because ya know, theplanet."
The Co-operative Bank has also teamed up with Unite to support its workforce to take part in the climate strikes around the country.
Muna Suleiman, Friends of the Earth campaigner, said most people wanted to fix the climate crisis but politicians needed to act.
She said: "Right when we need our leaders to step up, they continue to let us down."
Ms Suleiman added: "From filling the skies with more planes, to backing fracking in the UK and funding oil and gas projects abroad.
"That’s why we’re standing shoulder to shoulder with young people to call on our politicians to deliver emergency climate action now."