Thousands of people are protesting across the UK, with pupils leaving schools and workers downing tools as part of a global “climate strike” day.
Millions are taking part around the world with rallies in British cities including Glasgow, Manchester and London, urging “climate justice”.
Anna Taylor, 18, a co-founder of UK Student Climate Network said it was “very easy” to get people to show up.
Education Secretary Gavin Williamson said “every child should be in school”.
“They should be learning, they shouldn’t be bunking off and it’s very irresponsible for people to encourage children to do so,” he added.
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, Miss Ahmed 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.”
Demonstrations have also been organised in Cardiff, Edinburgh, Brighton, Newcastle, Bournemouth and Birmingham.
Students let off alarm bells at 13:00 BST to “raise the alarm” for the climate.
Labour leader Jeremy Corbyn addressed the climate change protest in Westminster, saying: “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 wanted every country to sign up to the Paris Agreement and, referring to President Donald Trump, said it was “disgraceful when you get a president of a major country like the US” who says they will walk away.
The Paris deal commits signatory nations to keeping global temperatures “well below” 2.0C (3.6F) above pre-industrial times and “endeavour to limit” them even more, to 1.5C.
Dozens of pupils from John Stainer Community Primary school in Brockley, south-east London, are among those taking part in protests in the capital.
Head teacher Sue Harte said “climate change is clearly a big issue” and “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.”
Hundreds of climate activists – including children in school uniform – have staged a mass “die in” in Belfast, where they lay down in the city centre.
One 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. We have to support the young people, they are the ones who started this strike,” she said.
“We are grieving for our future. I don’t feel happy about having children the way our climate is going.”
Extinction Rebellion ‘solidarity’
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 says it is supporting workers who want to join the action, while US clothing brand Patagonia is closing all of its stores and taking out adverts to back the protesters.
But in Norwich, protester Tiffany Wallace said her employer declined to give her time off work 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.”
The action follows school strikes inspired by activist Greta Thunberg.
The teenager, from Sweden, is set to join a rally planned in New York, where world leaders will meet at the UN next week to discuss climate change.
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”.