The Australian city of Sydney has kicked off Earth Hour, a worldwide annual event organised by the World Wildlife Fund to help combat global warming.

During the event, lights are switched off for one hour. Sydney's Opera House and Harbour Bridge went dark along with millions of homes. Harbour ferry horns blared to signal the energy-saving event.

This year it is being supported by 4,000 cities in a record 125 countries and some 1,200 famous landmarks, including Egypt's pyramids, London's Buckingham Palace, the Eiffel Tower, the Empire State Building and Beijing's Forbidden City, will turn off the lights. Earth Hour will end nearly 24 hours later in Samoa.

Earth Hour started in 2007 in Sydney and now enjoys widespread support both from the public and big businesses.

In December, two weeks of UN talks in the Danish capital Copenhagen failed to produce a binding commitment to limit global warming or set out concrete plans for doing so.