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”.
Students let off alarm bells at 13:00 BST to “raise the alarm” for the climate.
Energy minister Kwasi Kwarteng said their voices were “being heard”.
However, he said he could not “endorse children leaving school” to take part.
Jake Woodier, campaign co-ordinator at UK Student Climate Network said: “We understand it’s simply not feasible for many employees to take a day off to participate in a strike, but that doesn’t mean they can’t have a voice.”
Demonstrations have also been organised in Cardiff, Edinburgh, Brighton, Newcastle, Bournemouth and Birmingham.
Sebastian, a pupil from John Stainer Community School in Brockley, south-east London, said he joined the protests to help fight global warming.
He said: “They, the government, don’t understand that we’re going to go through it and they are not.”
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.”
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, of Barnet, north London, said: “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.”
Dozens of pupils from John Stainer Community Primary school in Brockley, south-east London, are among those taking part in the capital.
Head teacher Sue Harte said the school had decided to take part because “climate change is clearly a big issue” and “children need to know that they have a right to democratic protest”.
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.
She said: “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.
“We are grieving for our future. I don’t feel happy about having children the way our climate is going.”
Extinction Rebellion ‘solidarity’
Labour leader Jeremy Corbyn will address the group’s rally outside Westminster.
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.
The action follows earlier 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.
Mr Kwarteng told BBC Breakfast that although he could not support children leaving school, he did support “their energy, their creativity, and the fact that they have completely mastered these issues and take them very seriously”.