Two of the first landmarks to switch off their lights were the iconic Habour Bridge and Opera House in Sydney, Australia's largest city, with Hong Kong's waterfront, Egypt's pyramids, the Acropolis in Athens, Nigeria Falls and the Eiffel Tower in Paris, following suit.
Notable buildings and streets in China, Singapore, Thailand and the Philippines also switched off their lights, and others in London, Brazil and at the Vatican were doing the same later.
People joined in the campaign by holding candle lit picnics and concerts in a motion back to the days before the world was galvanised by electricity and its increasing use.
'Positive campaign'
It is the third year Earth Hour has been held after starting as a grass roots movement in Australia in 2007 and going global last year.
More than 50 million people were involved last year, according to WWF, the organisers.
About 2.2 million people were said to have turned their lights off in Sydney in 2007.
This year 3,929 cities and smaller habitations globally were said to be participating, with the WWF saying that they hoped for 1 bn participants.
"It is a very positive, hopeful campaign," Andy Ridley, the event's director, said in Sydney on Saturday.
"We want people to think, even if it is for an hour, what they can do to lower their carbon footprint and take that beyond the hour," Ridley said.
He said that he hoped the campaign would send a forceful message to the world's leaders to act on climate change.
It is the first time that China has taken part in the event, with 20 cities participating and the sight of Beijing's "Bird's Nest" Olympic Stadium being determined by the moon's rays.
China has overtaken the US as the largest greenhouse omitter in the world.
Climbers were also planning to raise an Earth Hour flag on the summit of Mount Everest, in Nepal, the highest point on the face of the world.
The event aims to encourage people to cut their energy use and reduce greenhouse gasses emitted via the burning of fossil fuels for electricity.
Scientists have said that greenhouse gasses contribute to climate change which could cause extensive flooding, droughts and disease around the world.