"It aims to raise awareness about the importance of protecting the environment and wildlife," Earth Hour organiser WWF Australia chief Dermot O'Gorman told AFP.
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".
"The time for denial is long past. We are losing not only our battle against climate change, but also our battle against the collapse of biodiversity," he said on Twitter.
Moscow's Red Square also fell dark and the Russian section of the International Space Station dipped its lights, the Ria Novisti news agency said.
Images from across Asia showed buildings including Kuala Lumpur's Petronas Towers, as well as the famous harbour skylines of Hong Kong and Singapore, blacking out to mark the occasion.
New York's Empire State Building emitted a "faint sparkle" to mark the hour.
And in Mexico City major monuments switched off their power including the Angel of Independence, while Colombia's third-largest city Cali saw activists light candles to form the "60+" logo.
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."
The cathedral of Guadalajara is pictured without lights during Earth Hour (AFP)
Threat of extinction
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 told AFP.
"I think... about how much one individual can make a difference."
