Australia's Sydney Harbour Bridge and 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 8:30pm 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 will go dark when the switches are flicked 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.

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."

Sydney was set to be swiftly followed by cities in Japan, Taiwan and China over the coming hours.

In Beijing, Olympic Park's two landmark monuments, the Bird's Nest and Water Cube, will spend 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 will turn off non-essential lighting.

In Taiwan, the presidential office will go dim and in the Philippines, more than 1,780 police stations and training centres will turn off all non-essential lights and electrical equipment.

"We will turn off the lights in the offices but not the lights in the perimeter because our detainees might escape," said Philippine national police spokesman Superintendent Agrimero Cruz.

In the Indian capital New Delhi lights at three iconic monuments, India Gate, Qutub Minar and Humayun's Tomb will be switched off.

"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 movement spreads across the planet buildings such as The Burj Khalifa in Dubai, the Eiffel Tower and Louvre in Paris, the cupola of St Peter's Basilica in the Vatican and Buckingham Palace in London will dim their lights.

The effort will also be 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.