From the Sydney Harbour Bridge to Egypt's pyramids, the Empire State Building in New York to Beijing's Birds Nest Stadium, the landmarks of the world were plunged into darkness on Saturday night as an exercise in energy saving symbolism unfolded across the globe.

As 8.30pm struck in each timezone, 4000 cities and towns in 88 countries switched off the lights to mark Earth Hour, a World Wildlife Fund sponsored campaign to highlight the threat of climate change.

Starting with the Sky tower in Auckland, New Zealand, crowds gathered around the world's most famous landmarks to see the lights dim. Two hours after NZ marked Earth Hour, Sydney's Harbour Bridge and Opera House went dark, followed shortly afterwards by Hong Kong's dazzling water front.

In all, around 371 landmarks worldwide were due to switch their lights off , including the Eiffel Tower, Niagara Falls, the Las Vegas casino strip and the Houses of Parliament in the UK. Honololu was due to be the last country to flip the switch.

The global event began in Australia in 2007, when 2.2 million people switched off their lights. Last year grew the movement grew to 400 cities and this year it has spread to 3,929 cities and villages around the world.

The research station in the Antarctic turned the lights off and even McDonald's took part, dimming the lights on the Golden Arches around America's mid-west.

China took part for the first time, cutting the lights at Beijing's Bird's Nest Stadium and Water Cube, the most prominent 2008 Olympic venues. In Bangkok, the prime minister switched off the lights on Khao San Road, a haven for budget travelers packed with bars and outdoor cafes.

In Rio de Janeiro, the iconic Christ the Redeemer statue that watches over the city of 6 million was darkened, along with the Copacobana beachfront.

"It is a very positive, hopeful campaign," Andy Ridley, the Earth Hour executive director, told reporters in Sydney as the city took its turn in the dark.

"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."

Mr Ridley said initial fears that the global economic crisis would dim enthusiasm for Earth Hour had dissipated as organisers realised it had the opposite effect.

"Earth Hour has always been a positive campaign; it's always around street parties, not street protests, it's the idea of hope, not despair. And I think that's something that's been incredibly important this year because there is so much despair around," he said.

He said he was aiming for one billion participants, hoping the event would send a resounding message to world leaders about significant emissions cuts.

United Nations Secretary Ban Ki-moon endorsed Earth Hour in a video message placed on YouTube, in which he described the event as "a way for the citizens of the world to send a clear message: they want action on climate change," he said.

But while environmentalists praised the countries that took part in Earth Hour, sceptics criticized the event as little more than empty symbolism. One critic, Danish professor Bjorn Lomborg claimed the use of candles during the dark hour could produce more emissions than electric lights.

The event comes on the eve of a UN climate change meeting, the first in a series of talks aimed at finding a replacement for the Kyoto Protocol.

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