Millions of people from some 170 countries and territories are expected to take part in the annual bid to highlight global warming caused by the burning of coal, oil and gas to drive cars and power plants.
The event, which originated in Sydney, has grown to become a worldwide environmental campaign, celebrated across all continents.
Conservation group WWF, which organises Earth Hour, said great strides had been made in highlighting the dire state of the planet.
"For that symbolic moment to turn into the global movement it is today, is really humbling and speaks volumes about the powerful role of people in issues that affect their lives."
More From This Section
In Sydney, many harbourside buildings switched off their lights for an hour from 8.30 pm local time as the call for action began rolling out across the world.
"I agree with the concept, 100 percent," said student Ed Gellert, 24, in Sydney.
"I think people probably avoid the fact that climate change is happening, so it's good to see the city grouping together to support Earth Hour."
In Singapore, some 200 organisations, including buildings along the city-state's iconic skyline, went dark to mark the occasion. Organisers said about 35,000 people watched performances and participated in a "carbon-neutral run" which saw some runners in panda and tiger costumes to raise awareness of wildlife protection.
And in Japan, Tokyo's famed Sony Building in Ginza extinguished its bright lights to honour the occasion.
Monuments including the Empire State Building, the Kremlin, Big Ben, the Leaning Tower of Pisa, the Eiffel Tower and Egypt's pyramids are all scheduled to switch off.
Lisbon will host a concert by candlelight, Singapore a "carbon-neutral run", and Tanzania will organise a tree-planting ceremony.
Homes and businesses were also being asked to join in, and individuals could commit to the cause on Facebook.
WWF said teams around the world would use Earth Hour this year to highlight climate issues most relevant to individual countries.
In South Africa, the focus would be on renewable energy while in China, WWF said it was working with businesses to encourage a shift towards more sustainable lifestyles.