In the United States, the lights went out at the Empire State Building in New York, the National Cathedral in Washington DC, and the Coca-Cola headquarters in Atlanta, among many other sites in the Eastern time zone.
Millions turned off lights and appliances for an hour 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, organised by the World Wildlife Fund.
Dan Forman, a spokesman for WWF in Washington, said: "I think it's great to see that hundreds of millions of people share this common value of lowering our carbon footprint."
Some 4,000 cities in more than 120 countries – starting with the remote Chatham Islands off the coast of New Zealand – voluntarily switched lights off on Saturday to reduce energy consumption, although traffic lights and other safety features were unaffected.
"We have everyone from Casablanca to the safari camps of Namibia and Tanzania taking part," said Greg Bourne, chief executive officer of WWF in Australia, which started Earth Hour in 2007 in Sydney before it spread to every continent.
The lights stayed on at the White House, US Capitol and the Lincoln and Washington monuments, although they were switched off at the Smithsonian Institution and the National Portrait Gallery.
In Europe, Italy's Leaning Tower of Pisa, the Arc de Triomphe in Paris and buildings across Germany went dark.
Prime Minister Gordon Brown, who switched off lights at Downing Street, said: "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."
Rome switched off the lights of the Trevi Fountain, its 18th-century landmark.