Millions of people worldwide are being urged to switch off lights for an hour, in what is described as the biggest climate change protest ever attempted.
The initiative, Earth Hour, was begun in Sydney two years ago by green campaigners keen to cut energy use.
Correspondents say the aim is to create a huge wave of public pressure to influence a meeting in Copenhagen later this year to seek a new climate treaty.
Critics describe the event as a symbolic and meaningless gesture.
The switch-off is expected to take place in more than 3,400 towns and cities across 88 countries, at 2030 in each local time zone.
Earth Hour was launched in 2007 as a solo event in Sydney, Australia, with more than two million people involved. Last year's event claimed the participation of 370 cities.
Full story [BBC]
