Lights have gone out all over the world, as millions shut down for "Earth Hour", an event in its fourth year which aims to highlight the threat of climate change.
Abroad, world-famous landmarks went dark including Sydney's Opera House and Harbour Bridge, Beijing's Forbidden City, The Pyramids and Eiffel Tower.
In Britain, Big Ben, the Houses of Parliament, St Paul's Cathedral and the London Eye were among the tourist hotspots that plunged into darkness, as was Manchester United's Old Trafford football ground.
Residents of Norway's Longyearbyen, the world's northernmost town, were braced for an influx of curious polar bears normally deterred by lights.
The residents voted - for the first time - that taking part was worth the risk.
The event was the biggest yet with 37 more countries taking part than last year, in the aftermath of the failed climate summit.
Despite December's fractious Copenhagen summit and recent controversy over climate science, public opinion still hopes for meaningful action to avert catastrophic global warming, according to Earth Hour founder Andy Ridley.
"There appears to be some fatigue to the politics around it. But people are far more motivated this year than they were last year," he said.
"Earth Hour is meant to cross geographic, economic, country boundaries," said Mr Ridley.
"It's one hour, one day, one year. We're not saving the planet by turning the lights off for one hour."
But, he added: "What you are doing is adding your voice to a global call for action."
Now run by the WWF, Earth Hour began in Sydney in 2007 when 2.2 million people switched off the lights in their homes, offices and businesses for 60 minutes to make a point about electricity consumption and carbon pollution.
The campaign went global the following year and this Saturday, more than 1,200 of the world's best-known sites will kill their lights at 8:30 pm local time in what organisers describe as a "24-hour wave of hope and action".
A raft of multinational companies including Google, Coca-Cola, Hilton, McDonalds, Canon, HSBC and IKEA have given their backing to Earth Hour 2010 and pledged to darken their offices worldwide in support.