Major cities and world landmarks plunged into darkness yesterday as a symbolic energy-saving exercise unfolded across the globe, calling for action to avert potentially devastating climate change.
The Egyptian pyramids at Giza, the Acropolis in Athens, Niagara Falls and the Eiffel Tower were set to switch off the electricity as countries worldwide prepared to join in 'Earth Hour'.
In London, demonstrators protesting in their thousands on poverty and environmental issues said that during Earth Hour they would raid companies and buildings that failed to switch off their lights.
The global event began dramatically as Sydney's iconic Opera House and Harbour Bridge plunged into darkness yesterday night, killing their lights for an hour, followed later by the glittering waterfront of Hong Kong.
Millions of people turned out in the harbourside Australian city. Melbourne, the country's second-largest city, came to life with a pedal-powered concert, and others enjoyed moonlit picnics and barbecues.
The global grassroots movement began in Sydney two years ago, when 2.2 million people switched off their lights. Earth Hour has since grown to include 3,929 cities, villages and localities across the globe.
"It is a very positive, hopeful campaign," Andy Ridley, the event's director, told reporters in Sydney.
"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."
Scientists have warned that global warming caused by burning fossil fuels on a massive scale could devastate the planet, hitting the poorest countries hardest with floods, droughts and disease.
Some 371 landmarks were powered down worldwide, including also the Vatican, the Empire State Building, the Las Vegas casino strip and Beijing's 'Bird's Nest' Olympic stadium.
The event ended hours later in Honolulu, capital of the US state of Hawaii. Ridley said he hoped the event sent a resounding message to world leaders about significant emissions cuts.
Sceptics criticise the event as little more than empty symbolism, with one Danish professor claiming the use of candles during the hour could produce more emissions than electric lights.
"Even if a billion people turn off their lights, the entire event will be equivalent to switching off China's emissions for six short seconds," said Bjorn Lomborg, director of the Copenhagen Consensus Centre think-tank.
But Ridley said the overwhelming response to the event - including participation by emerging, high-emissions economies such as Brazil, India and China - showed there was a mandate for tough action on climate change.
A UN-led conference in the Danish capital later this year is meant to approve a new global warming treaty for after 2012, when the Kyoto Protocol for cutting carbon emissions expire.