New York's Empire State Building, the Eiffel Tower in Paris, and the Sydney Opera House were plunged into darkness 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 was to observe from the International Space Station countries around the world turn off the lights for 60 minutes and post photos.
From Sydney's sparkling harbor to Egypt's Tahrir Square, and huge neon billboards that normally light up New York's Times Square, thousands of cities and businesses opted to go dark across some 150 countries and territories.
"Let us stand together to make of our world a sustainable source for our future as humanity on this planet," former South African president Nelson Mandela urged in a message sent to Twitter, using the hashtag "#EarthHour."
Organizers said the official YouTube channel for Earth Hour was seeing some 20,000 visits per minute as the day rolled on.
Among US East Coast landmarks hitting the light switch was the under-construction "Freedom Tower" in lower Manhattan, steadily rising above the New York skyline in place of the destroyed World Trade Center towers, as well as the headquarters of the United Nations.
Secretary General Ban Ki-moon said the UN was turning off its lights "in solidarity with the men, women and children -- 20 per cent of all humankind -- who live with no access to electricity," adding that the Earth Hour movement was "a symbol of our commitment to sustainable energy for all."
The towering Washington National Cathedral also plunged into darkness to raise awareness about the urgent need to arrest global warming.
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, harborside buildings went dark, along with most big office buildings as some Sydneysiders picnicked on the harbor foreshore by moonlight.
Japan's Tokyo Tower interrupted its sunset-to-midnight lighting to take part, as organizers said the Earth Hour was an opportunity to pray for last year's earthquake and tsunami disaster.
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.
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 neighborhood 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.
As the hour pushed east across the globe, Rio de Janeiro's joined the movement, dimming its lights on central monuments and tourist attractions, as 130 cities across Brazil also joined in to go dark.
The effort was also being observed from 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.
Explore further: Ice in Arctic seas shrinks to sixth-lowest recorded