Hundreds of millions of people across the globe will turn off their lights for 60 minutes on Saturday night starting at 8:30pm local time in a symbolic show of support for "Earth Hour" campaign against climate change.
Many of the world's most iconic attractions, including Sydney Opera House, the Empire State Building and the Eiffel Tower will take part.
"What started as an event in Sydney in 2007 with two million people has now become a tradition across the country and across the world," Dermot O'Gorman, head of WWF[World Wildlife Fund]-Australia, said.
"It's now an organic, people-powered movement... which is fantastic."
Last year more than 150 countries participated in the event which saw some of the world's most iconic landmarks dim, and this year the movement has spread to Palestine, Tunisia, Suriname and Rwanda.
Newcomers to be plunged into darkness include Copenhagen's Little Mermaid, the statue of David in Florence and Cape Town's Table Mountain.
In Australia, where Earth Hour originated with an appeal to people and businesses to turn off their lights for an hour to raise awareness about carbon pollution, the Sydney Opera House and Sydney Harbour Bridge will be among the first sites to participate globally.
'Switch off for good'
This year Earth Hour Australia is asking participants to "switch off for good" and move to renewable energy. As part of the push Sydney Opera House will not go dark at 8:30 pm (0930 GMT) but will instead glow a deep green.
With restaurant diners eating by candlelight, Outback communities going dark and iconic buildings standing in shadows, O'Gorman believes Earth Hour has played a part in drawing attention to energy use.
"We've always heard anecdotally that it has made people change their actions and look at their impact on the planet in a more considered way," he said.
"Earth Hour has always been about empowering people to realise that everybody has the power to change the world in which they live, and thousands of people switching to renewable energy is a perfect example."
Sydney's lights out will be followed by countries across the globe, with the Petronas Towers in Malaysia, the Bird's Nest in Beijing, and the Burj Khalifa all participating.
Russia will turn off the lights on the Kremlin for the first time later on Saturday as part of worldwide events for the Earth Hour campaign against climate change, officials said.
In China, Shanghai's famous Bund will turn off its lights while in the central city of Wuhan, the Yangtze River bridge will be plunged into darkness.
In Japan, daily illuminations of the city's signature Tokyo Tower will be switched off, with visitors able to pedal bicycles to generate power to illuminate an egg-shaped art work.