The lights are being switched off around the world at 8.30pm (local time in each location) on Saturday, to mark the 10th annual Earth Hour and to draw attention to climate change.
The Earth Hour initiative began in Australia in 2007 as a grassroots gesture by the World Wildlife Fund (WWF) Australia against man-made carbon dioxide emissions linked to a warming planet.
In 2017, the campaign will involve the switching off of electric lights for an hour in 7,000 cities across 172 countries, at 8.30pm local time, with the aim of highlighting the need to act on climate change, and saving a few megawatts of power in the process.
Among the famous buildings and structures taking part in Australia are Sydney's Opera House, the Harbour Bridge, Luna Park, Town Hall, and Sydney Tower Eye.
Internationally the list includes some of the world's best known skyscrapers and historic buildings including the world's tallest building, the Burj Khalifa in Dubai, London's Big Ben and Houses of Parliament, the Colosseum in Rome, Istanbul's Blue Mosque, the Eiffel Tower, Moscow's Kremlin and Red Square and the Pyramids of Egypt.
While the organisers of Earth Hour say they do not audit results of the energy-saving initiative, the group has commissioned research indicating up to one in four Australians gets involved.
WWF says Earth Hour can take credit for various environmental initiatives, such asthe 2013 declaration of a 3.4-million-hectare marine park in the waters off Argentina, the planting of a forest in Uganda and a ban on soft plastics in the Galapagos Islands.