Protests are taking place across the UK, with pupils leaving schools and workers downing tools as part of a global “climate strike” day.
Rallies are taking place in cities including Glasgow, Manchester and London, urging “climate justice” and “an end to the age of fossil fuels”.
Students and workers have also been encouraged to let off alarm clocks across the country at 1300 BST.
Energy minister Kwasi Kwarteng said their voices were “being heard”.
However, he said he could not “endorse children leaving school” to take part.
Other demonstrations have also been organised in Cardiff, Edinburgh, Brighton, Newcastle, Bournemouth and Birmingham.
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”.
Labour leader Jeremy Corbyn will address the group’s rally outside Westminster at about 13:15 BST.
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 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 said the protesters’ voices were being heard but he could not “endorse children leaving school”.
He told BBC Breakfast: “What I do support is their energy, their creativity, and the fact that they have completely mastered these issues and take them very seriously.”