Australia's Opera House was the first of many global landmarks to go dark on Saturday as Earth Hour got underway with hundreds of millions of people around the world set to switch off their lights.
The Eiffel Tower in Paris and the world's tallest building in Dubai will also have their lights turned off for 60 minutes as part of the remarkable event in which people reflect in darkness about how to fight global warming.
"The amount of power that's saved during that time is not really what it's about," Earth Hour co-founder and executive director Andy Ridley told AFP in Sydney, where the movement began in 2007.
"What it is meant to be about is showing what can happen when people come together."
Ridley said a record 134 countries or territories were on board for the event, which organisers have dubbed the world's largest voluntary action for the environment.
Other landmarks that will go dark for the hour are Times Square in New York, Beijing's Bird's Nest stadium that hosted the 2008 Olympics, the London Eye and Brazil's Christ the Redeemer statue.
The lights will be turned off from 8:30pm local time around the world.
It kicked off in the Pacific, Fiji, New Zealand and Australia, and was then set to roll into Asia, Europe, Africa and the Americas as it followed the descending sun.
Originally designed as a symbolic act to make people aware of everyday energy use, organisers this year are asking people to also commit to an action, large or small, that they will carry through the year to help the planet.
As part of the longer-term commitment, Dalian city in northeastern China will spend $ 1.5 billion planting 340 million trees and Chengdu city in the southwest will make up to 60,000 bicycles available for public rental.
Ridley said Earth Hour, organised by global environment group the WWF, this year would also focus on connecting people online so they could inspire each other to make commitments to help protect the environment.
In Japan, reeling from a huge earthquake and tsunami that struck this month, several thousand people and a hotel-turned-evacuation centre in the northeast were expected to mark Earth Day.
"People in Japan will have a special feeling this year when they turn the switches off," WWF spokeswoman Hideko Arai told AFP.
"We will not only think about climate change but also the people who need energy in the disaster-hit areas," she said.
"We want to show our support for disaster victims."