Towns and cities around the world are turning out the lights for an hour to highlight the threat of climate change.
Sydney was the first major city to begin "Earth Hour", when at 2000 (0900 GMT), lights went out on landmarks like the Opera House and Harbour Bridge.
Some major Bangkok shopping malls were later plunged into darkness, while street lights in Manila turned off.
Toronto, Chicago, San Francisco and Dublin were among 27 cities due to follow suit at 2000 local time.
Critics have dismissed the event as a gimmick that will have little effect.
Australians marked Earth Hour by holding candle-lit beach parties, dinners and poker games, while traditional Aboriginal torchlight performances were also held.
And in New Zealand, thousands of homes and more than 100 business in Christchurch turned out the lights.
WWF Thailand said the switch-off in Bangkok saved 73.34 megawatts of electricity, which would have produced 45.8 tonnes of carbon dioxide.
The initiative began in Sydney last year when an estimated two million residents took part, cutting energy usage by more than 10% for the hour.
Organisers expect hundreds of towns and cities across the world to take part in the event and hope some capitals not officially involved, such as London and Rome, will mark the event by dimming lights on some landmarks.
In its own contribution, the Google web search engine is putting a dark background on its homepage.
Organisers insist the aim of Earth Hour is to show that communities care passionately about climate change and want to keep up the pressure on governments to act decisively.
Andy Ridley of the WWF, which is behind the initiative, says interest has been immense.
"We're aware of villages in Norfolk in England that are doing Earth Hour and we're aware of the big cities like Chicago and Sydney that are doing it," he told the BBC.
Australia is one of the world's worst per capita emitters of greenhouse gases and many believe recent droughts and floods are the result of man's destabilising influence on the climate, the BBC's Phil Mercer reports from Sydney.
New Prime Minister Kevin Rudd has made the environment one of his priorities, signing up to the Kyoto Protocol on tackling climate change soon after he took office.