Lights went off in thousands of cities and towns across the world for the annual Earth Hour campaign, which is aiming to raise money via the internet for local environmental projects.
Sydney's Opera House and Harbour Bridge were among the first landmarks around the world to dim their lights for 60 minutes during Saturday's event, organised by conservation group WWF.
An estimated 7,000 cities and towns from New Zealand to New York were taking part.
Hong Kong's stunning waterfront skyline was unrecognisable on Saturday evening, with the city's tallest skyscraper, the International Commerce Centre, stripped of the vast light show usually wrapped around its 118 stories.
Neon switch off
Blazing neon signs advertising some of the world's largest brands were shut off, leaving the view of the heavily vertical southern Chinese city peppered only with tiny lights from buildings' interiors.
Earth Hour partnered with payments giant PayPal to allow donors to contribute to specific projects from Russia and India to Canada and Indonesia, using Asian fundraising site Crowdonomic.
Earth Hour chief executive Andy Ridley said before the lights went off in Singapore that the event had moved beyond symbolism to concrete action.
"If you want to get real social change you need to have symbolism," he told the AFP news agency. "We are seeing some really big outcomes."
Projects under the "Earth Hour Blue" crowdfunding scheme - which aim to raise more than $650,000 in total - include a turtle centre in Italy and funding for forest rangers in Indonesia.
The event is being marked in more than 150 countries, organisers said, estimating that thousands of cities and towns would have taken part by the time the ceremonies began in Singapore.
Earth Hour will see other landmarks including the Empire State Building in New York, the Eiffel Tower in Paris and the Kremlin in Moscow switch off their lights for an hour starting at 8:30 pm local time on Saturday.