The Sydney Opera House, the Eiffel Tower and Moscow's Red Square were among the world landmarks to go dark on Saturday, as part of a global campaign to raise awareness about the impacts of climate change.
Earth Hour, which started in Australia in 2007, was observed by millions of supporters in 187 countries, who turned off their lights at 8.30pm local time in what organisers described as the world's "largest grassroots movement for climate change".
"It aims to raise awareness about the importance of protecting the environment and wildlife," Earth Hour organiser WWF Australia chief Dermot O'Gorman said.
In Paris, the Eiffel Tower plunged into darkness as President Emmanuel Macron urged people to join in and "show you are willing to join the fight for nature".
UN Secretary-General Antonio Guterres said the event "comes at a time of huge pressure on people and planet alike".
"Resources and ecosystems across the world are under assault. Earth hour is an opportunity to show our resolve to change."
With global temperatures the highest on record, O'Gorman said this year's theme was the impact of climate change on biodiversity and plant and animal species.
"More than half of plant and animal species face local extinction in some of the world's most naturally rich areas in biodiversity by the turn of this century if we continue along the current path that we are trending in terms of global warming," he said.
Species at risk include Australia's green turtles, black-flanked rock wallabies and koalas, as well as the Adelie penguin colonies in Antarctica, the conservation group said in a report it commissioned that was published in the science journal Climatic Change.
The analysis, released last week, said key biodiverse sites around the world projected to be most affected by localised extinction include the Amazon, the world's largest tropical rainforest, and southern Africa's Miombo Woodlands.
While the lights-off event is a symbolic gesture, Earth Hour has led successful campaigns over the past decade to ban plastics in the Galapagos Islands and plant 17 million trees in Kazakhstan.
Sydneysider Dianna Ali, who was having dinner with family as the lights went off in the city, said the initiative had made her more aware of the impact of her lifestyle on the planet's health.
"Since Earth Hour started, it's made me more conscious of how much power I'm using," she said.
"I think... about how much one individual can make a difference."