Cities around the world have been 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.
Lights were then turned off in Bangkok, Manila, Budapest, Copenhagen and Dublin as those cities joined the effort.
Toronto, Chicago, San Francisco were among several hundred cities in 35 countries taking part in the event.
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 estimate that 380 towns and cities are taking part.
In Britain, 26 councils dimmed lights, as did Prince Charles' private residence, Highgrove House and Winchester cathedral.
On the south coast, Brighton turned off the lights on its pier, and in London - which was not officially involved - lights were turned down at City Hall.
In the Irish capital, Dublin, the floodlights were turned off at the Custom House, the home of the Environment Department.
But in the city's financial district many lights were left on.
"The banks should have embraced this wholeheartedly and they didn't," said Cathy Flanagan, an Earth Hour organiser in the city. "But it's a start. Maybe next year."
In Copenhagen, people enjoyed a rare chance to gaze at the night sky.
"It's not often you can see the stars in a city," said local Earth Hour spokeswoman Ida Thuesen.
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.
Internet search engine Google turned its normal white homepage black.
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.