As each time zone reaches the appointed hour, skylines go dark and landmarks dim, from a Manila shopping mall to Berlin's Brandenburg Gate and the Empire State Building in New York.
Some 4,000 cities in more than 120 countries -- starting with the remote Chatham Islands off the coast of New Zealand -- were voluntarily switching off Saturday to reduce energy consumption, though traffic lights and other safety features would be unaffected, organizers said.
"We have everyone from Casablanca to the safari camps of Namibia and Tanzania taking part," said Greg Bourne, CEO of World Wildlife Fund in Australia, which started Earth Hour in 2007 in Sydney before it spread to every continent.
In Katmandu, Nepal -- where electricity supplies aren't constant -- protesters unable to turn out lights held a candlelight vigil, while in the Maldives the state broadcaster ceased transmission for an hour to mark the event, WWF said.
Organizers hoped the event would put pressure on global lawmakers to push for clear progress on agreeing a binding international pact to reduce greenhouse gas emissions.
In Europe, Italy's Leaning Tower of Pisa and buildings across Germany and a host of nations went dark. Amsterdam cut the lights at most city buildings including Schiphol Airport, Artis Zoo and the Amsterdam Arena.
While France's Eiffel Tower dimmed its lights only briefly for security reasons -- switching off for five minutes, rather than a full hour -- other city landmarks, including the Arc de Triomphe were in darkness for the duration of the protest.
"It's saying to our politicians -- you can't give up on climate change," said WWF spokeswoman Debbie Chapman in the U.K.
Buckingham Palace and the British Parliament building were scheduled to go dark to support the campaign, along with other famed London landmarks including St. Paul's Cathedral and the Royal Albert Hall, as well as Edinburgh Castle in Scotland.
"Tackling climate change is urgent and vital to both safeguard our environment and our children's future. We can make a difference if we act now and act together," said British Prime Minister Gordon Brown, who switched off lights at his Downing Street residence in London.
Rome switched off the lights of the Trevi Fountain, the 18th-century landmark where many tourists flip a coin in hopes of coming back to the city. State-TV RAI showed the fountain that was immortalized by Federico Fellini in "La Dolce Vita" falling dark.
Moscow's iconic and imposing State University, perched on a hill overlooking the city, all but disappeared into the darkness as the city took part in the protest. The gigantic Luzhniki Stadium nearby also went black, as did the skyscraping Ukraina Hotel downtown.
Officials in Russia said they hoped to beat last year's national participation figure of more than 6 million people in 20 cities across the vast country, and it appeared they were on track. Around 40 cities had taken part by the time the event reached Moscow, including Far East spots like Kamchatka and Siberia's Irkutsk, which joined their Asian neighbors in switching off earlier. Restaurants in Vladivostok held a so-called Candle Evening, promoting Earth Hour as a chance for romance.
Sweden turned out lights at the government's headquarters in Stockholm, the golfball-shaped Ericsson Globe arena, royal castles and streets in several towns, including popular skiing resort Are.
In Dubai, the world's tallest building, the Burj Khalifa went dark in solidarity with the protest.
Organizers say there's no uniform way to measure how much energy is saved worldwide during the event, but hope global participation will send a message to leaders that people worldwide are worried about global warming.
"What we're still looking for in this coming year is a global deal that encourages all countries to lower their emissions," said Andy Ridley, a WWF worker in Sydney who came up with the idea of Earth Hour in a pub with friends.
He said he hoped this year's event would inspire world leaders to strive for a climate change deal that would hold nations to rules for reducing greenhouse gas emissions -- blamed for global warming.
"China is going to have to be a big part of that, but so is every other major economy," Ridley said.
Giant panda Mei Lan led events in 30 Chinese cities, walking onto a platform amid dimming lights in her enclosure at the Chengdu Panda Breeding Research Center in the southwestern province of Sichuan, said Chris Chaplin of WWF in China. Lights were also turned off in Beijing's imperial palace known as the Forbidden City.
Some 1,000 towns and cities in the Philippines switched off their lights, and a rock concert and street party were held at a Manila Bay mall complex, according to WWF-Philippines.
Taiwan's Presidential Palace and at least 20 Taipei skyscrapers went dark, while hundreds of Taiwanese placed candles beside a Taiwan map formed by energy-saving LED lights at a square outside the city hall.
Researchers at the Davis Station, in Antarctica, also joined the campaign -- shutting off lights at the base.
"Tonight, hundreds of millions of people are raising their voices by turning out their lights. It is a simple act, but a powerful call to action," said WWF Director-General, James Leape.
Last year, some 88 cities took part in Earth Hour, which is backed by the United Nations as well as global corporations, nonprofit groups, schools, scientists and celebrities.