Millions were expected to turn off lights and appliances for an hour from 8:30 p.m. in a gesture to highlight environmental concerns and to call for a binding pact to cut greenhouse gas emissions. This year's was the fourth annual Earth Hour, organized by the World Wildlife Fund.
"I think it's great to see that hundreds of millions of people share this common value of lowering our carbon footprint," said Dan Forman, a spokesman for WWF in Washington.
Some 4,000 cities in more than 120 countries -- starting with the remote Chatham Islands off the coast of New Zealand -- voluntarily switched off Saturday to reduce energy consumption, though traffic lights and other safety features were unaffected, organizers said.
"We have everyone from Casablanca to the safari camps of Namibia and Tanzania taking part," said Greg Bourne, CEO of WWF in Australia, which started Earth Hour in 2007 in Sydney before it spread to every continent.
Other sites expected to participate in the U.S. were businesses on the glittering Las Vegas strip and the Mount Rushmore presidential monument in South Dakota. The lights stayed on at the U.S. Capitol and the Lincoln and Washington monuments.
In Europe, Italy's Leaning Tower of Pisa, the Arc de Triomphe in Paris and buildings across Germany went dark. Amsterdam planned to cut the lights at most city buildings including Schiphol Airport, Artis Zoo and the Amsterdam Arena.
"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. 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.
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.
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.