The environmental group WWF urged governments, businesses and households to turn back to candle power for at least 60 minutes starting at 8 p.m. wherever they were.
The campaign began last year in Australia, and traveled 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."
Earth Hour officials hoped 100 million people would turn off their nonessential lights and electronic goods for the hour. Electricity plants produce greenhouse gases that fuel climate change.
Several U.S. cities also plan symbolic blackouts or dimmings of monuments, including the Golden Gate Bridge in San Francisco.
In Chicago, lights illuminating the Sears Tower's famous antennas flicked off. The red and white marquee outside Wrigley Field went dark. The stripe of white light around the top of the John Hancock Center disappeared.
Lights on more than 200 downtown Chicago buildings along with the state Capitol dome in Springfield were dimmed Saturday night.
"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," Richard Moss, a member of the Nobel Peace Prize-winning Intergovernmental Panel on Climate Change and the climate change vice president for WWF, said in Chicago on Saturday.
Enthusiasm for Earth Hour was even higher north of the U.S. border with 100-thousand Canadians out of a total of 300-thousand people worldwide registered online for the event - putting the country among top participants anywhere.
Thousands jammed Nathan Phillips Square in downtown Toronto for a concert headlined by Nelly Furtado, who sang her hit song "Turn Out The Light," while lights on the city's C-N Tower were doused.
In Montreal, even the bulbs on the cross atop Mont Royal went out.
About 4,000 Canadian businesses, including hotels, stores and restaurants, signed on to take part.
The campaign began last year in Australia, and traveled this year from the South Pacific to Europe in cadence with the setting of the sun. Several U.S. cities also planned symbolic blackouts or dimmings of monuments, including at the Golden Gate Bridge in San Francisco.
In Sydney, where an estimated 2.2 million observed the blackout last year, officials said it appeared at least as popular this time, involving untold candlelight dinners and beach-bonfire parties. The city's two architectural icons, the Opera House and Harbour Bridge, faded to black.
Last year's shutdown produced an estimated cut of 10.2 percent in Australia's carbon emissions for that hour.
More than two dozen cities and 300 towns across the globe planned their own smaller, largely symbolic switch-offs.
Lights went out at the famed Wat Arun Buddhist temple in Bangkok, Thailand; shopping and cultural centers in Manila, Philippines; several castles 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.
Activists gathered outside one of Dublin's most impressive floodlit buildings, the riverfront Custom House, and cheered as the lights went out. The building houses the Environment Department, run by a Green Party minister.
But next door, 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.
Ireland's more than 7,000 pubs elected not to take part - in part because of the risk that Saturday night revelers could end up smashing glasses, falling down stairs, or setting themselves on fire with candles.
Likewise, much of Europe - including France, Germany, Spain and European Union institutions - planned nothing to mark Earth Hour.
Internet search engine Google lent its support to Earth Hour by blackening its normally white home page and challenging visitors: "We've turned the lights out. Now it's your turn."
For more information, visit the organizers' Web site at http://www.earthhour.org/.