Thousands of school pupils worldwide have abandoned classrooms for a day of protest against climate change.
India, South Korea, Australia and the US are among the countries where teenagers are already on strike.
The day of action is expected to embrace about 100 countries. They are inspired by Swedish teenager Greta Thunberg, who protests weekly outside Sweden's parliament.
Scientists say tougher measures are needed to cut global warming.
The Paris climate agreement of 2017 committed nearly 200 countries to keeping global temperatures "well below" 2.0C (3.6F) above pre-industrial times and to striving for a maximum of 1.5C.
The globally co-ordinated children's protests - promoted through posts on Twitter and other social media - have been going on for several months.
On Thursday Greta Thunberg's campaigning earned her a Nobel Peace Prize nomination.