From the Sydney Opera House to Rome's Colosseum to the Sears Tower's famous antennas in Chicago, floodlit icons of civilization went dark Saturday for Earth Hour, a worldwide campaign to highlight the threat of climate change.

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.

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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.

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 Center. 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," 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.

Workers in Phoenix turned out the lights in all downtown city-owned buildings for one hour. Darkened restaurants glowed with candlelight in San Francisco while 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.

Lights also 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.

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.

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."