The Eiffel Tower, the Acropolis and the Sydney Opera House were plunged into darkness on Saturday for the annual Earth Hour campaign, leading a global effort to raise awareness about climate change.
In a twist to this year's Earth Hour, Dutch astronaut Andre Kuipers will observe from the International Space Station countries around the world turn off the lights for 60 minutes from 8:30 pm local time and post photos.
From Sydney's sparkling harbour to Egypt's Tahrir Square and New York's Empire State Building, thousands of cities opted to go dark in some 150 countries and territories.
"From the Sydney Opera House it was fantastic," said Marni Ryan, from organiser WWF Australia. "We had the skyline of Sydney all out."
The Pacific island nation of Samoa was the first to make the symbolic gesture, with New Zealand's city landscapes later dramatically darkened as lights on buildings such as Auckland's Sky Tower were cut.
In Australia, where the event was conceived, harbourside buildings went dark, along with most big office buildings as some Sydneysiders picnicked on the harbour foreshore by moonlight.
Japan's Tokyo Tower interrupted its sunset-to-midnight lighting to take part, as organisers said the Earth Hour was an opportunity to pray for last year's earthquake and tsunami disaster.
But the just completed 634-metre Tokyo Sky Tree, the world's tallest communications tower, loomed with blinking lights as it prepared to open to the public in May.
"We will ask the tower's operator to turn off the lights next year," WWF spokeswoman Akiko Ishihara said.
In Hong Kong the city's skyscrapers turned out their lights dimming the usually glittering skyline. Tourists and locals snapped pictures, although many were unaware of what was behind the switch-off.
Since it began in Sydney in 2007, Earth Hour has grown to become what environmental group WWF says is the world's largest demonstration of support for action on carbon pollution.
A total of 5,251 cities took part in 2011, as the movement reached 1.8 billion people in 135 countries, it says. Newcomers to the worldwide initiative include Libya and Iraq.
"Earth Hour 2012 is a celebration of people power -- the world's largest mass environmental event in support of the planet," said chief executive of WWF-Australia Dermot O'Gorman.
"And we're seeing hundreds of millions of people in different countries around the world take actions to go beyond the hour in support of positive actions for climate change and the planet."
In Beijing, Olympic Park's two landmark monuments, the Bird's Nest and Water Cube, were spending an hour in darkness.
And in Singapore, 32 malls -- many located in the glittering Orchard Road shopping belt -- and more than 370 companies including luxury brands such as Louis Vuitton and Armani turned off non-essential lighting.
In Taiwan, the presidential office went dim and in the Philippines, more than 1,780 police stations and training centres turned off all non-essential lights and electrical equipment.
In the Indian capital New Delhi lights at three iconic monuments, India Gate, Qutub Minar and Humayun's Tomb, were switched off, while in Mumbai people gathered on the streets to light candles.
"We have a lot of power cuts in our neighbourhood so we're used to going without power, but my kids want to turn out the lights for Earth Hour -- they've been learning about energy conservation at school," Delhi mother-of-two Sangeeta Dayal said.
As the initiative passed through the time zones, Dubai's Burj Khalifa, the world's tallest tower, went dark in the United Arab Emirates.
In debt-stricken Greece, lights were turned off at the Acropolis and at town halls across the country.
In Paris, darkness fell across a string of monuments and buildings including Notre Dame cathedral, and the Bastille and old Garnier opera houses.
The lights at the Eiffel Tower, however, were only turned off for five minutes for security reasons.
The effort was being observed by the International Space Station, where Kuipers is hoping to share photos and live commentary as he watches from above.
"There is no better way to raise awareness for the future of the most beautiful planet in the universe," he said earlier this month.