Billions of people from more than 130 countries are expected to turn off their lights on Saturday for Earth Hour 2011, the event's organizer said on Thursday.

Earth Hour, organized by environment group World Wildlife Fund (WWF), was first started in Sydney, Australia, in 2007 with 2.2 million people taking part to turn off their lights in aid of the environment.

By 2009, up to 1 billion citizens across the world had taken part.

According to Earth Hour spokesman Winston Su, 133 countries had signed up for the event this year, with 13 of them taking part for the first time.

"We've built global awareness of the event," Su said. "The aim was always to not use non-essential lighting."

All Australian cities are taking part in the 2011 event. The Opera House and Harbor Bridge in Sydney will power down for an hour.

Australian Prime Minister Julia Gillard recorded a message for Earth Hour, saying she was "going beyond the hour" by bringing in a carbon price.

"As well as switching off lights for Earth Hour, people who take part are pledging themselves to climate change action - big or small - for the next 12 months," Gillard said.

"My commitment for this year is simple: I will do everything in my power to deliver a carbon price ... I believe 2011 is the year Australia will choose action. Everyone taking part in Earth Hour this year is helping to make sure this is so. Earth Hour is a wonderful example of Australian creativity and confidence at its best."

Further afield, international landmarks to be powered down include Times Square and the Empire State Building in New York, the Eiffel Tower in Paris, and the tallest building in the world, Dubai's Burj Khalifa. In Asia, cities that have signed up include Bangkok, Hong Kong, New Delhi and Mumbai.