The Sydney Harbour Bridge was plunged into darkness for the Earth Hour environmental campaign on Saturday night, among the first landmarks around the world to dim their lights for the event.
The campaign, initiated by the World Wide Fund for Nature, involved millions of people around the world switching their lights off at 8.30pm, in their time zone, for one hour.
Lights went out in some 7,000 cities and towns for Earth Hour, which this year aimed to raise hundreds of thousands of dollars for environmental projects.
At Parliament House in Canberra, almost 4,000 candles spelt out "Lights out for the Reef", a nod to the focus of this year's campaign in Australia.
World landmarks including the Eiffel Tower and the Kremlin have taken part, switching off their lights for 60 minutes.
Landmarks which participated in London included Buckingham Palace, Tower Bridge, Trafalgar Square, St Paul's Cathedral and Wembley Stadium.
In Jordan, a candlelit march was held along one of the oldest streets in Jabal Amman and the lights of Al Karak Castle were turned off.
Gwanghwamun Square, in the South Korean capital of Seoul, was also plunged into darkness.
In the United States, landmarks switching off include the Golden Gate Bridge in San Francisco, Times Square and the Empire State Building in New York and the Space Needle in Seattle.
Some of the hotels and casinos on the famous Las Vegas strip are also turning off their gaudy displays.
Australian event focuses on Great Barrier Reef
Earth Hour began in 2007 in Sydney, but the idea quickly spread around the world and hundreds of millions of people are estimated to have turned their lights off for the event last year.
Earth Hour national manager Anna Rose says there were more events organised around the country on Saturday night than in previous years.
"Millions of Australians participated in Earth Hour, with a call for stronger action on climate change to protect our reef," she said.
"Earth Hour is calling for Australia to have a higher renewable energy target and also to have a more ambitious target for cutting carbon pollution.
"It's quite beautiful when people turn off their lights in Earth Hour to know that they are joining with people in 154 countries."
This year's event focused on the Great Barrier Reef.
Dr Selina Ward, a coral reef biologist who supports the Earth Hour campaign, says the reef is one of the most vulnerable places on Earth to climate change and the impact is heartbreaking.
These include the bleaching of coral, increasing ocean and sand temperatures, more extreme storm damage to the reef and rising sea levels.
"Our reef is running out of time, but those of us alive today can be the ones to help save it," she said.
Dr Ward says the reef currently supports more than 63,000 livelihoods, including people working in the tourism industry and the fishing industry.
The event has drawn some criticism, including from Danish political scientist Bjorn Lomborg, who argues it does little for the real problem of global warming and diverts resources from other issues.
"This celebration of darkness sends the wrong message," Dr Lomborg said in a statement this week.
"While more than a billion people across the globe make a symbol of foregoing non-essential electrical power for one hour a year, another 1.3 billion people across the developing world will continue to live without electricity as they do every other night of the year."