lights for an hour in a global call for swift action on climate change.

Chatham Island, the largest of a tiny group of Pacific islands 800 kilometres (500 miles) southeast of New Zealand, unofficially began Earth Hour by switching off its diesel generators at 0645 GMT, or 8:30 pm local time.

The 25-hour energy-saving marathon officially began in Sydney shortly after 0930 GMT with a spectacular switch to darkness for an hour before spreading across the world for more than 80 countries to take part at 8:30 pm local time.

Millions in the harbourside city appeared to observe the hour without light to mark Earth Hour, a global grassroots movement that began in Sydney two years ago, when 2.2 million people switched off their lights.

It 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."

Thousands lined the harbour for a rare glimpse of blacked-out Sydney, while the centre of Melbourne, Australia's second-largest city, came to life with a pedal-powered concert and others enjoyed moonlit picnics and barbecues.

Some 371 landmarks will power down the world over including the Vatican, Niagara Falls, the Eiffel Tower, the Empire State Building, the Las Vegas casino strip, the Acropolis and Beijing's "Bird's Nest" Olympic stadium.

Hong Kong's glittering waterfront and more than 1,500 buildings were to dim their lights, including many of the city's skyscrapers such as the Bank of China Tower, HSBC's headquarters and the giant International Finance Centre 2.

The city would also suspend its daily light show for the first time.

Mountaineers were even expected to raise an Earth Hour flag on the 8,848-metre (29,000-foot) summit of Everest, the planet's highest point, with the event to end in Honolulu, capital of the US state of Hawaii.

Ridley said he was aiming for one billion participants the world over, in a move he hoped would send 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 actually produce more emissions than electric lights.

"Even if a billion people turn off their lights this Saturday 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 -- would show 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 the period after 2012, when the Kyoto Protocol's obligations to cut carbon emissions expire.