The Eiffel Tower, the Sydney Opera House and even the ancient Acropolis in Athens were plunged into darkness for an hour as part of a global campaign to raise awareness about climate change and its impact on the planet's vanishing plant and animal life.
The 13th edition of Earth Hour, organised by green group WWF, saw millions of people across 180 countries turn off their lights at 8.30pm to highlight energy use and the need for conservation.
"We are the first generation to know we are destroying the world. And we could be the last that can do anything about it," the charity said.
"We have the solutions, we just need our voices to be heard."
WWF-Australia CEO Dermot O'Gorman told AFP that "Earth hour still is the world's largest grassroots movement for people to take action on climate change".
"It's about individuals taking personal action but joining with hundreds of millions of people around the world to show that not only do we need urgent action on climate change but we need to be protecting our planet," he added.
Dozens of companies around the world said they would take part in this year's campaign, which also saw Singapore's skyline go dark and Hong Kong turn off the lights along the Victoria Harbour.
Other global landmarks that flicked the switch included Eiffel Tower in Paris, New York's Empire State Building, Dubai's Burj Khalifa, the Acropolis in Athens and the Kremlin building in Moscow.
The event comes after some of the most dire warnings yet on the state of Earth's natural habitat and species.
WWF's own "Living Planet" report in October said 60% of all animals with a backbone - fish, birds, amphibians, reptiles and mammals - had been wiped out by human activity since 1970.
Another study said a recent decline in bugs that fly, crawl, burrow and skitter across still water -- fuelled by deforestation, urbanisation and the rise of commercial farming - was part of an unfolding mass extinction event, only the sixth in the last half-billion years.
Last year, Earth Hour was observed in more than 7,000 towns and cities in 187 countries, according to the organisers.
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.