Protests are expected across the UK, with pupils leaving schools and workers downing tools in a bid to urge more government action on climate change.
It’s part of a global “climate strike” day, which started in Australia earlier, where organisers said around 300,000 people took part.
They are urging “climate justice” and “an end to the age of fossil fuels”.
Energy minister Kwasi Kwarteng said their voices were being heard but did not “endorse children leaving school”.
It follows earlier school strikes inspired by activist Greta Thunberg.
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The teenager, from Sweden, has described the turnout in Australia earlier as “incredible”.
She is set to join a later rally planned in New York, where world leaders will meet at the UN next week to discuss climate change.
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.
Labour leader Jeremy Corbyn will address the UK Student Climate Network’s Global Climate Strike in Westminster at about 13:15pm.
Mr Kwarteng told BBC Breakfast protesters’ voices were “being heard” but insisted the time spent in school was “incredibly important”.
He added: “What I do support is their energy, their creativity, and the fact that they have completely mastered these issues and take them very seriously.”