The movement, a mass symbolic gesture supporting environmental action, began in Australia five years ago and has grown to include 6000 cities and some of the world's most famous landmarks.
Sydney's Opera House and Harbour Bridge, Big Ben, the Great Wall of China and the Tokyo Tower will all be plunged into darkness for 60 minutes at 8.30pm (local time).
"Earth Hour 2012 is a celebration of people power; the world's largest mass event in support of the planet," Dermot O'Gorman, from organisers WWF, said in Sydney on Saturday.
He anticipates about 40 per cent of Australians will take part.
Corporations around the world will also turn off their lights and millions of households are expected to take part by switching off lights and appliances.
Libya's Martyrs' Square, a focal point for the 2011 revolution, will join Earth Hour for the first time.
Parts of Iraq and Afghanistan will also take part and Dutch astronaut Andre Kuipers will turn off lights on the International Space Station.
Earth Hour has attracted criticism in the past, including claims in the UK's Telegraph newspaper in March 2010 that it could actually lead to an increase in carbon emissions.
"It's not designed and it would be foolhardy to believe that switching a light switch (off) will save the planet," Earth Hour co-founder Todd Sampson said.
"It was originally done to raise awareness; to get people to think about it and then take action in many different ways.
"I think scepticism is part of the debate; it moves everything forward."
NSW Police have urged householders to be vigilant about their property as the lights go off during Earth Hour.
"I'd probably say just to remind everyone to remain vigilant," a spokesman said.
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