Thousands of people around the UK have joined a global climate change protest, with pupils walking out of schools and workers downing tools to demand action.
Millions around the world are taking part in the "climate strike" day, with rallies in British cities including Glasgow, Manchester and London.
Anna Taylor, a co-founder of UK Student Climate Network, said it was "very easy" to persuade people to show up.
Education Secretary Gavin Williamson said "every child" should be in school.
He added: "They should be learning, they shouldn't be bunking off and it's very irresponsible for people to encourage children to do so."
Student Jessica Ahmed, 16, emailed her school to warn that she would be joining the protests instead of being in class.
Speaking at a protest in Westminster, she said: "School is important but so is my future.
"If politicians were taking the appropriate action we need - and had been taking this action a long time ago when it was recognised the world was changing in a negative way - then I would not have to be skipping school."
Organisers estimated that around 100,000 people attended a rally in central London, while more than 20,000 were thought to have marched in Edinburgh and 10,000 in Brighton.
In Belfast, organisers put the turnout at between 3,000 and 4,000, with young people taking over the Cornmarket area of the city centre and staging a "mass die-in".
And in Birmingham, around 3,000 protesters, including hundreds of children, gathered in the city's Victoria Square before marching through nearby streets.
UK Student Climate Network said more than 200 events had been organised across the country.
Labour leader Jeremy Corbyn told young people at the Westminster rally: "You and a whole generation have brought [climate change] centre stage and I am absolutely delighted about that.
"If we're going to sustain this planet we need to get to net zero emissions a lot, lot quicker than 2050 [the government's target]," he said.
He wants every country to sign up to the Paris Agreement, which commits signatory nations to keeping global temperatures "well below" 2.0C (3.6F) above pre-industrial times.
Referring to President Donald Trump, Mr Corbyn said it was "disgraceful when you get a president of a major country like the US" who says they will walk away.
The global protests come ahead of a summit at the UN next week that will urge countries to do more to avoid the worst effects of climate change.
Dozens of pupils from John Stainer Community Primary school in Brockley, south-east London, were among those taking part in protests in London.
Head teacher Sue Harte said "children need to know that they have a right to democratic protest".
Sebastian, a pupil at the school, said he joined the protests to help fight global warming.
"They, the government, don't understand that we're going to go through it and they are not," he said.
Eight-year-old Sohan and Nayan, five, also from south-east London, joined protesters with their mother, Celine.
Sohan said: "We want to save our planet and we hope that marching will help."
At the Belfast protest, Extinction Rebellion activist Lorraine Montague, from County Tyrone, was dressed as a swan to highlight the threat of climate change to wildlife.
"Our climate is at crisis point and the government is not doing anything about it," she said.
"We are grieving for our future. I don't feel happy about having children, the way our climate is going."
In Edinburgh, demonstrators - the majority of them young people - chanted and sang as they marched from the Meadows to Holyrood Park near the Scottish Parliament.
They carried placards reading "Scotland, you're not too wee to change the world" and "If you were smarter, I would be in school".
Extinction Rebellion, which organised its own climate and environment protests in the UK earlier this year, said it stood "in solidarity" with those taking part.
It added that its members were joining the strikes and holding their own events, including a choir and "kids' space" in Victoria Tower Gardens, Westminster, and outside King's College London.
Some trade unions, including the TUC, the University and College Union and Unite, are supporting members who take part in the "strikes".
Co-operative Bank said it supported workers who want to join the action, while US clothing brand Patagonia closed all of its stores and took out adverts to back the protesters.
But in Norwich, protester Tiffany Wallace said her employer declined to give her time off work to join demonstrators "because they didn't think it was important".
"The worst thing they can do is fire me," said the 33-year-old.
"I don't feel I should compromise my own values and integrity and what's important, so I can make money for a business."
Energy minister Kwasi Kwarteng said he could not "endorse children leaving school" to take part in the protests.
But he said he did support "their energy, their creativity, and the fact that they have completely mastered these issues and take them very seriously".
London Mayor Sadiq Khan, Green Party MP Caroline Lucas and Liberal Democrat leader Jo Swinson all supported the walkout for the Youth Strike 4 Climate campaign.
The Metropolitan Police said seven people were arrested for public order offences, but overall the day ran "smoothly".
Commander Dave Musker said a "tiny minority of protesters" tried to block Lambeth Bridge, a key route for emergency services to St Thomas' Hospital. One other man was arrested on the bridge on suspicion of discharging a flare in a public place.
The global action follows a long-running series of school strikes initially inspired by activist Greta Thunberg.
The teenager, from Sweden, is also playing a role in the day's events and is set to join a rally planned in New York, where world leaders will meet at the UN next week.