The campaign began last year in Australia, and travelled this year from the South Pacific to Europe to North America in cadence with the setting of the sun.
"What's amazing is that it's transcending political boundaries and happening in places like China, Vietnam, Papua New Guinea," said Andy Ridley, executive director of Earth Hour. "It really seems to have resonated with anybody and everybody."
Canadians across the country embraced powerlessness, dining by candlelight, counting stars or swaying in the dark to acoustic music as they marked the country's first Earth Hour.
At 8 p.m. local time, enthusiastic participants turned off lights and appliances for the 60-minute event that swept around the globe in what was possibly the world's largest voluntary power outage since the invention of the incandescent bulb.
So many people did their part in Ontario that demand for power fell by 900 megawatts during the hour - a drop of just over five per cent.
"Canada is a leader in this," Mike Russill, head of World Wildlife Fund Canada, told a crowd of thousands who jammed a downtown Toronto square for the hour.
"Climate change is the biggest threat to this planet and your individual actions count."
About 100,000 Canadians out of a total of 300,000 people worldwide registered online for the event -putting this country among top participants anywhere.
Organizers said they represented just the tip of an iceberg that surfaced for the hour itself, sinking those cynics who argued the event was just a publicity stunt that would do little to combat global warming.
Earth Hour officials hoped 100 million people would turn off their non-essential lights and electronic goods for the hour. Electricity plants produce greenhouse gases that cause climate change.
In Chicago, lights on more than 200 downtown buildings were dimmed Saturday night, including the stripe of white light around the top of the John Hancock Centre. The red-and-white marquee outside Wrigley Field also went dark.
"There's a widespread belief that somehow people in the United States don't understand that this is a problem that we're lazy and wedded to our lifestyles. (Earth Hour) demonstrates that that is wrong," said Richard Moss.
He's a member of the Nobel Peace Prize-winning Intergovernmental Panel on Climate Change and the climate change vice president for the WWF.
Workers in Phoenix, Ariz., America's fifth-largest city, turned out the lights in all downtown city-owned buildings for one hour.
San Francisco concluded the display as the Golden Gate Bridge, Coit Tower and other landmarks extinguished lights for an hour.
New Zealand and Fiji were first out of the starting blocks this year.
And, in Sydney, Australia - where an estimated 2.2 million observed the blackout last year - the city's two architectural icons, the Opera House and Harbour Bridge, faded to black against a dramatic backdrop of a lightning storm. Australian organizers said national power usage dropped by more than three per cent during Earth Hour. Figures released Sunday showed that 59 per cent of Sydney participated in the event.
Lights also went out at the famed Wat Arun Buddhist temple in Bangkok, Thailand; shopping and cultural centres in Manila, Philippines; several centres in Sweden and Denmark; the parliament building in Budapest, Hungary; a string of landmarks in Warsaw, Poland; and both London City Hall and Canterbury Cathedral in England.
Greece, an hour ahead of most of Europe, was the first on the continent to mark Earth Hour.
On the isle of Aegina, near Athens, much of its population marched by candlelight to the port. Parts of Athens itself, including the floodlit city hall, also turned to black.
In Ireland, where environmentalists are part of the coalition government, lights-out orders went out for scores of government buildings, bridges and monuments in more than a dozen cities and towns.
But the international banks and brokerages of Dublin's financial district blazed away with light, illuminating floor after empty floor of desks and idling computers.
"The banks should have embraced this wholeheartedly and they didn't. But it's a start. Maybe next year," said Cathy Flanagan, an Earth Hour organizer in Dublin.
Much of Europe - including France, Germany, Spain and European Union institutions - planned nothing to mark Earth Hour.