The Sydney Opera House and the Harbour Bridge were plunged into darkness for an hour Saturday to raise awareness about climate change and its impact on the planet's vanishing biodiversity.
The 13th edition of Earth Hour, organised by the green group WWF, will see millions of people across 180 countries turn off their lights at 8:30 pm local time 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 have said they will join in this year's switch-off.
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 that 60 percent of all animals with a backbone -- fish, birds, amphibians, reptiles and mammals -- had been wiped out by human activity since 1970.
Another dataset confirmed the depth of an unfolding mass extinction event, only the sixth in the last half-billion years.
Paris's Eiffel Tower, New York's Empire State Building, Dubai's Burj Khalifa and the Acropolis in Athens are among the 24 global landmarks that will take part in Earth Hour.
Last year's event was observed in more than 7,000 towns and cities in 187 countries, according to 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.