Lights across the city of London were switched off on Thursday night for an hour on Thursday night to encourage London''s three million households to conserve energy. Similar campaigns have been held in Sydney, Paris and Rome to raise awareness on global warming.

Among the London landmarks that were plunged into darkness were The Houses of Parliament, Buckingham Palace and Piccadilly Circus. It was the first time the lights at Piccadilly Circus had voluntarily been switched off since World War II.

Organised by the radio station Capital 95.8FM and supported by Friends of the Earth, the ''Lights Out, London'' campaign was aimed at shutting off all non-essential lighting between 9:00 and 10:00 PM British Time.

Earlier in the day, environment minister Ben Bradshaw urged all Londoners to switch off their lights during the blackout. "Excessive illumination contributes to climate change and you may like to join in, if you are in London tonight, the London Lights Out campaign," Bradshaw said in the House of Commons.

The campaign organisers said if all Londoners were to have switched off their lights, the city would save 380 tonnes of CO2 in just one hour, enough to fill more than 2,000 double-decker buses.

The event was based on Earth Hour, a similar, hour-long mass switch-off earlier this year in Sydney that involved 65,000 households and 2,000 businesses and landmarks, including the Opera House and Sydney Harbour Bridge.

Environmental groups, say, that the majority of carbon dioxide emissions in heavily urbanised cities come from buildings, so switching off lights and appliances that aren''t needed can play a significant role in tackling climate change, apart from saving money.

They are awarded each year to deserving projects that can benefit local communities and also be expanded to boost sustainable development.