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.
Demonstrations have also been organised in Newcastle upon Tyne, Bournemouth, Edinburgh, Cardiff and Belfast.
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