Sydney, Austrailia ? The Sydney Opera House’s gleaming white-shelled roof was darkened Saturday night along with much of the rest of Australia’s largest city, which switched off the lights to register concern about global warming.

Lights from skyscrapers that normally illuminate Sydney, Australia's skyline are off Saturday. Businesses and homeowners switched off the lights to draw attention to global warming.

The arch of Sydney’s other iconic structure, the harbor bridge, was also blacked out, along with dozens of skyscrapers and countless homes in the 4 million-strong city, in an hourlong gesture that organizers said they hoped would be adopted as an annual event by cities around the world.

Mayor Clover Moore, whose officials shut down all nonessential lights on city-owned buildings, said Sydney was “asking people to think about what action they can take to fight global warming.”

Restaurants throughout the city held candlelit dinners, and families gathered in public places to take part in a countdown to lights out, sending up a cheer as lights started blinking off.

Buildings went dark one by one. Some floors in city skyscrapers remained lit, and security and street lights, those at commercial port operations and at a sports stadium, stayed on.

“It’s an hour of active, thoughtful darkness, a celebration of our awakening to climate change action,” said Oscar-winner Cate Blanchett, who attended a harborside function to watch the event.

While downtown was significantly darker than normal, the overall effect, as seen in television footage from overhead helicopters, was that the city’s patchwork of millions of tiny lights had thinned, not disappeared.

“We were expecting a big difference straight away, but it was just a little bit,” said Sonja Schollen, who took sons Harry and James to a park to watch the skyline, joining other families.

Organizers hope Saturday’s event – which about 2,000 businesses and more than 60,000 individuals signed up for online – will get people to think about regularly switching off nonessential lights, powering down computers and other simple measures they say could cut Sydney’s greenhouse gas emissions by 5 percent this year.