The World Wildlife Fund's seventh annual Earth Hour is being observed in major cities around the globe, including Sydney, Paris and Moscow, the BBC reported.
The WWF's campaign encourages cities to turn off non-essential lights for an hour at 8:30 p.m. local time.
In Sydney, power to the city's skyline was cut at 09:30 GMT. The city's Opera House was lit in green to symbolise renewable energy, the BBC reported.
Russia is participating the Earth Hour for the first time this year and will be cutting power to the Kremlin and 90 other landmark buildings in Moscow.
Lights on the Eiffel Tower in Paris will be shut off for Earth Hour, along with 400 other monuments across France, Radio France Internationale reported.
"The message is not because you switch off your lights for an hour that it will change something, but the thing is that ... the major preoccupation of people all over the planet is climate change. It's very important to show that the people are ready to change. And they want to say that to the politicians. You can change individually, you can change your way of life, but you can also change at the global level, at the state level," said Isabelle Autissier, the president of the World Wildlife Fund in France.
Earth Hour was first conceived by the WWF in Australia in 2005 in response to scientific data that showed impact of climate change on the globe, CBC News reported. The first Earth Hour was held in Sydney two years later in 2007