Hundreds of landmarks from Berlin's Brandenburg Gate to the Great Wall of China went dark on Saturday night as part of a global effort to highlight climate change.
Earth Hour, held on the last Saturday of March every year, began as a Sydney-only event in 2007. The city's Harbour Bridge and Opera House were dimmed again this year.
Australia is among the first countries to flick off the light switches each year.
The Empire State Building turns off its lights in this combined image showing before (left) and during (right) Earth Hour in New York. Photo: AFP
In New Zealand, Sky Tower in Auckland and the parliament buildings in Wellington switched off two hours earlier.
Tokyo Tower was also dimmed and in Hong Kong buildings along Victoria Harbour also went dark.
The WWF, the global environmental group which organises the event, says the number of countries and territories participating has grown to 147 from 135 last year.
Lights dim on world landmarks for Earth Hour 2012
This combo of pictures shows the Leaning Tower of Pisa with the lights on (left) and with the lights off (right) during Earth Hour on March 31, 2012, in Pisa. Photo: AFP
"Global warming is a big issue," said Rudy Ko of Taiwanese environmental group Society of Wilderness. "Everybody can help reduce the problem by turning the lights off."
Ko said children should invite parents "to turn the lights off, go out, go to the parks to do some exercise and enjoy some family time instead of watching TV or playing video games".
In Europe, 5000 candles were lit in the form of a globe in front of Berlin's Brandenburg Gate before city officials switched off the monument's lighting.
More than 230 monuments and major gathering points in Paris dimmed lights for an hour - including Notre Dame Cathedral, the Arc de Triomphe and fountains and bridges over the Seine.
However, the Eiffel Tower went dark for only five minutes "for security reasons".
Big Ben, Buckingham Palace, Tower Bridge and St Paul's Cathedral were among the London landmarks to go dark. The Savoy Hotel planned to light its lobby, bars and restaurants with candles.
"Let us stand together to make of our world a sustainable source for our future as humanity on this planet," the Nelson Mandela Centre of Memory said in a tweet.
Across the Nordic nations, government buildings and municipalities joined in, including Stockholm's royal castle and the Swedish capital's globe-shaped sports arena.
In Sweden's second-largest city, Goteborg, the main boulevard was bathed in an hour's darkness.
People launched paper lanterns into the air in St Petersburg, Russia.
Washington's National Cathedral and New York's Empire State Building also took part.
Libya, Algeria, Bhutan and French Guinea were among the nations participating in Earth Hour for the first time.