WELL over a billion people turned off their lights for Earth Hour on Saturday night, in 150 nations and territories around the world, making it the largest voluntary event of its kind.
Governments and citizens in 6525 cities, towns and municipalities joined in - about 24 per cent more than the previous peak last year, according to the organisers, the environment group WWF.
''It's been five years since the first Earth Hour in 2007, and the extraordinary growth this year shows hope for action for the planet is not diminishing - instead it's growing,'' Earth Hour's executive director, Andy Ridley, said.
''We hope that this strong signal gives leaders, whether of government organisations or communities, even more confidence to accelerate our journey towards a sustainable future.''
The final estimate of the number of people who took part will take another month to firm, while polls are collated from around the world. Surveys last year estimated the event reached 1.8 billion people.
About 40 per cent of the population of Sydney is thought to have taken part this year, a similar number to last year.
The Harbour Bridge and Opera House were the first Sydney icons to go dark, followed by dozens of city office buildings in the CBD.
The glittering face of Luna Park, which recently upgraded to more energy-efficient LED lights, switched off promptly at 8.30pm, while the lights of Coney Island followed a few minutes later.
Thousands of monuments and major public buildings had their lights dimmed or switched off, including the Great Wall of China, Big Ben in London and the world's tallest building, Burj Khalifa in Dubai.
Earth Hour dominated social media traffic, with one video about the event registering more than 4 million views on YouTube at the weekend.
On Twitter, the words ''earth hour'' were among the most popular terms used in English at the weekend, peaking as the 8.30pm local time event swept through Asia, and topping trends on Spanish-language Twitter feeds as it passed across South America.
The former South African president Nelson Mandela posted the most popular tweet under the Earth Hour tag. It read: ''Let us stand together to make of our world a sustainable source for our future as humanity on this planet.''
The Facebook website was used by citizens in war-torn Libya, northern Iraq and Kurdistan to co-ordinate a series of Earth Hour events.
Earth Hour is supported by Fairfax Media, the publisher of the Herald.