Hundreds of landmark buildings and millions of ordinary homes were switching off their lights on Saturday as the annual Earth Hour moved around the globe in what was dubbed the world's largest voluntary action for the environment.
Australia's Opera House was the first of many global landmarks to go dark as the event got under way, as hundreds of millions of people prepared to follow suit to enhance awareness of energy use and climate change.
Others in their turn included Beijing's Bird's Nest stadium that hosted the 2008 Olympics, the Eiffel Tower in Paris, the London Eye Ferris wheel, Times Square in New York and Brazil's Christ the Redeemer statue.
Most were switching off their floodlighting, advertising signs and other illuminations for an hour from 8:30pm local time.
"The amount of power that's saved during that time is not really what it's about," Earth Hour co-founder and executive director Andy Ridley told AFP in Sydney, where the movement began in 2007.
"What it is meant to be about is showing what can happen when people come together."
Ridley said a record 134 countries or territories were on board for this year's event, which organisers have dubbed the world's largest voluntary action for the environment.
Organisers this year also asked people to commit to an action, large or small, that they will carry through the year to help the planet.
For example, Dalian city in northeastern China will spend 1.5 billion dollars planting 340 million trees and Chengdu city in the southwest will make up to 60,000 bicycles available for public rental.
The event kicked off in the Pacific, Fiji, New Zealand and Australia, rolling into Asia, Europe, Africa and the Americas as it followed the descending sun.
Ridley said Earth Hour, organised by global environment group the WWF, this year would also focus on connecting people online so they could inspire each other to make commitments to help protect the environment.
Nearly 600,000 people had "liked" Earth Hour's official Facebook page by the time the event began in the Asia Pacific, while hundreds of tweets with the #earthhour hashtag were appearing on Twitter every few minutes.
In Australia, organisers said an estimated 10 million people, nearly half the population, took part, with Sydney Harbour Bridge another of the landmarks to go dark.
Hong Kong's neon waterfront dimmed, while in Singapore all decorative lights were switched off and non-critical operational lights lowered at Changi Airport for an hour.
The airport said the effort would result in energy savings equivalent to the total amount of electricity consumed by a four-room apartment over three months.
In Japan, which is reeling from a huge earthquake and tsunami that struck this month, several thousand people and a hotel-turned-evacuation centre in the northeast marked Earth Day.
"People in Japan will have a special feeling this year when they turn the switches off," WWF spokeswoman Hideko Arai told AFP ahead of the switch-off.
"We will not only think about climate change but also the people who need energy in the disaster-hit areas," she said. "We want to show our support for disaster victims."
The March 11 twin disaster left more than 27,000 people dead or missing and hundreds of thousands more homeless, sheltering in emergency facilities.
In Paris a minute's silence was to be observed for Japan as the city of light went dark, with illuminations switched off at the cathedral of Notre Dame, City Hall, the two opera houses and many bridges, fountains and public places.
Another 129 French towns and cities were also taking part in EarthHour.
In Russia some 30 cities were joining in, from Petropavlovsk-Kamchatsky, the most easterly city on the Kamchatka peninsula, through Moscow to Murmansk in the far north.
Moscow was to turn off floodlighting on more than 70 buildings and bridges, including the 540-metre (1,780-foot) television tower and the 32-storey Moscow State University building.
In Athens monuments being darkened included the Acropolis, the parliament building, the presidential palace and at the temple of Poseidon near the city.
United Nations Secretary General Ban Ki-moon backed Earth Hour, urging people to celebrate the shared quest to "protect the planet and ensure human well-being".
"Let us use 60 minutes of darkness to help the world see the light," he said.
British Prime Minister David Cameron said sharing responsibility was the key to fighting climate change, describing Earth Hour as "a huge symbol of global solidarity, an inspiring display of international commitment".
Ridley said he never expected the Earth Hour movement to become so large.
"We didn't imagine right at the beginning... it would be on the scale that it is now. And the fact that it is so cross cultural, beyond borders and race and religion," he said.