Major cities and world landmarks have been plunged into darkness as part of a campaign to raise awareness of potentially devastating climate change.

Homes and offices in major cities and landmarks turned their lights off for an hour on Saturday to promote energy-saving in response to the threat of climate change cuased by global warming.

Two of the first landmarks to switch off their lights were the iconic Habour Bridge and Opera House in Sydney, Australia's largest city, with Hong Kong's waterfront, Egypt's pyramids, the Acropolis in Athens, Nigeria Falls and the Eiffel Tower in Paris, following suit.

Notable buildings and streets in China, Singapore, Thailand and the Philippines also switched off their lights, and others in London, Brazil and at the Vatican were doing the same later.

People joined in the campaign by holding candle lit picnics and concerts in a motion back to the days before the world was galvanised by electricity and its increasing use.