Two of the first landmarks to switch off their lights were the iconic Habour Bridge and Opera House in Sydney, Australia's largest city.
The WWF-run event officially began when New Zealand's Chatham Islands was the first of more than 100 nations and territories to turn off the power, in a rolling event around the globe that ends just across the International Dateline in Samoa 24 hours later.
The energy-saving event is supported by 4,000 cities in a record 125 countries and includes 1,200 landmarks from the Forbidden City to Egypt's pyramids and the Las Vegas Strip.
"From Brazil to America, to Canada, all the way down to Australia, Japan and India - it's a really diverse set of countries taking part this year," Andy Ridley, the Earth Hour executive director, said.
The rolling wave of darkness was intended to boost the environmental movement after disappointing UN talks in Copenhagen in December.
Beijing's Forbidden City and Bird's Nest Stadium were among the participants along with other cities in China, which is the world's biggest carbon polluter and appointed giant panda Mei Lan its Earth Hour "ambassador".
Hong Kong's neon waterfront dimmed, as did office buildings in Jakarta, Seoul and Tokyo.
The Japanese city of Hiroshima turned off the lights at 30 sites, including its Peace Memorial, set in one of the few buildings to survive an atom bomb attack during World War II.
More than 100 students arranged candles to spell out "Peace and Eco", on the ground near the dome.
About 300 participants gathered in Jakarta to light hundreds of candles and lanterns set out in the shape of the number 60 - representing the 60 minutes of Earth Hour.
In Delhi and Mumbai, lights were switched off at shops, hotels, the Rashtrapati Bhavan presidential residence, the 17th-century Red Fort and the Swaminarayan Akshardham temple complex, one of India's largest Hindu places of worship.
A rock concert took place at New Delhi's India Gate to highlight the environmental cause and Bollywood celebrities joined the calls for action.
In December, two weeks of UN talks in Copenhagen failedto produce a binding commitment to limit global warming or set out concrete plans for doing so, in a setback for the environmental movement.
"As responsible citizens of this planet, it's extremely crucial for us to address the colossal problem of climate change through ensuring responsible action," Abhishek Bachchan, a Bollywood star, said.
|UN talks in Copenhagen failed to produce a binding commitment to limit global warming
India is expected to be among the countries hit hardest by rising temperatures and changes in rainfall patterns, with experts warning such problems could affect food security and displace communities.
In Europe, London's Big Ben and Manchester United's Old Trafford football ground were set to take part, along with Paris's Notre Dame cathedral and the Trevi Fountain in Rome.
In America, some 30 states were on board, with Mount Rushmore, San Francisco's Golden Gate Bridge and Chicago's 110-storey Sears Tower all due to go dark.
But in Bangkok, city authorities were ordered to halt their Earth Hour campaign for security reasons as anti-government protestersheld a major rally.
The initiative, Earth Hour, begun in Sydney in 2007 by green campaigners keen to cut energy use - and enjoys widespread support both from the public and big business, including Google, Coca-Cola and fast-food giant McDonald's.
This year, even users of ubiquitous Twitter and Facebook can show their support with special applications that turn their displays dark.