Australia in 2007, the environmental movement Earth Hour asks households and landmarks across the globe to switch off their lights.
The event, organised by the conservation charity WWF, takes place throughout Saturday, with buildings going dark between 8.30pm and 9.30pm local time in each country.
The WWF is not promoting public gatherings because of the coronavirus pandemic, but wants supporters to join in at home through online events.
Earth Hour wrote on its official Twitter page last week: “At its core#EarthHour has always been about the power of the people.”
“During these times, whilst we may not be able to get together in person, we can still symbolically stand in solidarity with millions of others across the world from the comfort of our own homes,” the post added.
It is thought that more than 7,000 cities in 170 countries took part last year.
Katie White, executive director of advocacy and campaigns at WWF-UK, said: “These are really unprecedented times, and I know a lot of people are looking for ways to connect and feel connected.”
“In this global health crisis, now is a pivotal time for us to work together to safeguard our future and the future of our planet,” she added.
Additional reporting from PA