Hours later, Beijing's most prominent Olympic venues, the Bird's Nest and Water Cube, went dark. China is taking part for the first time, with major cities like Hong Kong, Shanghai and Guangzhou also dimming their lights.
Other locations due to take part this time include Edinburgh Castle in Scotland, St Peter's Basilica in Rome, Paris' Eiffel Tower, the Egyptian Pyramids and New York's Empire State Building.
Fast-food giant McDonald's has pledged to dim its "golden arches" at 500 locations, while celebrities such as actress Cate Blanchett and Archbishop Desmond Tutu have promised support.
UN Secretary-General Ban Ki-moon backed the initiative in a video posted this month on the event's YouTube channel.
"Earth Hour is a way for the citizens of the world to send a clear message," he said. "They want action on climate change."
People are invited to provide blogs and short video clips on how they spend their time.
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