Sydney's famous skyline (top image) was shrouded in darkness
At 1930 (0930 GMT) the city's skyline dimmed and normally bright landmarks like the Opera House and the Harbour Bridge went dark.
The so-called Earth Hour is supported by the New South Wales government, environmental groups and businesses.
Sydney hopes the event will make a very big statement on climate change.
The city of four million people is aiming to become the first anywhere to achieve a blackout on this scale.
The BBC's Phil Mercer, in Sydney, says by and large Sydney had never been this dark.
He says lights were off in the majority of the central business district's office blocks and large parts of the suburbs were also in darkness.
Co-operation
Greg Bourne of environmental group WWF, one of the driving forces behind Earth Hour, said the big switch off took months to plan.
"The logistics is really quite amazing in the sense every tower block is owned by one company, maybe leased by another company, have 10 tenants in and a manager and working through all of these people has been fantastic."
Many restaurants signed up and planned to serve diners by candlelight.
The owner of the Newtown Hotel, which says it is Australia's oldest gay bar, told the BBC before the blackout that they would have fun while trying to send a serious message.
"Sometimes drag queens [female impersonators] do look better in the dark anyway," said Roger Zee.
"It's up to the patrons. They'll actually have their own torches so they'll be able to light up the drag queens on the stage themselves."
Organisers want to encourage Australians to conserve energy and to think carefully about what they can do to cut pollution.
Every day millions of lights and computers are left on in deserted office blocks as well as in apartments and houses.
Campaigners have said that simply switching them off could reduce Sydney's greenhouse gas emissions by 5% over the next year.
Australia is one of the world's largest per capita producers of carbon dioxide and other gases that many scientists believe are helping to warm the earth's atmosphere.
A selection of your comments
We turned our household lights off for Earth Hour, although realistically it didn't make any difference to the amount of greenhouse gases generated. I think most Sydneysiders saw Earth Hour as a way of expressing our growing awareness of the need to do something about the factors contributing to climate change rather than a 'fix' for any part of the problem. To that extent, it was a success.
Phil Keeffe, Sydney, Australia
Why doesn't John Howard and his Premiers sign up to Kyoto instead of pulling off publicity stunts. Perhaps this will get the residents of Australia to think and put pressure on their 'leaders' to do so.
Andrew, Brisbane (soon to be Edinburgh)
Have we already forgotten that Paris tried this too on the occasion of the International Panel on Climate Change conference just a month ago? The Eiffel tower lights went off, but most ordinary people did nothing. At least for Paris we got numbers on the amount of power saved. What for Sydney?
Robert Cailliau, Prevessin, France
Well done Sydney! Every city should follow. I am not that old and I remember when public lights in cities were off from 11pm to 6am!
gerome mortelecque, Toronto Canada
One hour? how much difference will that make to climate change.
adrian, ashby de la zouch
Kudos to Sydney! Hong Kong made a similar attempt last year but unlike its counterpart in New South Wales, the government refused to participate in the campaign, claiming that it would give "adverse publicity to Hong Kong as an international metropolis". Let's hope that the precedent set by Sydney will help persuade other cities, and their governments, to follow suit.
C. Chan, Hong Kong
It's true. Sydney has never been so dark. Fantastic! Thank you to WWF for organising such an event. Australian's are cyring out for ways to get involved and make a difference.
Kath Eggleston, Sydney
I live in Sydney and I can assure you that the 'blackout' didn't happen. The lights on the bridge and opera house were turned off but the publicity stunt was completely ignored by the vast majority of ordinary people here. The vast majority of people here did not turn off their lights.
Nick Mallory, Sydney
Amazing! Very glad to hear of something like this happening. Mr. Mallory, even if most people didn't turn of their lights, I'm sure you would agree that the 'stunt' at least got the world talking and citizens of other large cities wanting/working towards something similar. Nicely done Sydney!
Raj Trivedi, Houston, Texas USA
Good on the Aussies! Why isn't every country doing this???
Lee, Bournemouth
I'd like to commend Australia for taking such a bold initiative. One can only imagine the scale of positive impact this can have if all the world's major cities observed voluntary blackouts even for a few minutes if not hours.
Adithya Hanbal, Michigan, USA
Sydneys Opernhaus bei Nacht

 Mit einer so genannten „Earth Hour“ (Stunde der Erde) wollte die australische Millionenstadt Sydney am vergangenen Abend ein Zeichen gegen die globale Erwärmung setzen. In der Vier-Millionen-Einwohner-Stadt blieben am Samstag um 19:30 Ortszeit (09:30 Uhr UTC) für eine Stunde die Lichter aus. Die Stadt lag völlig im Dunkeln. Sowohl Geschäftshäuser, Büros und Straßenlaternen als auch Restaurants und Privathäuser beteiligten sich an der Aktion. In vielen Restaurants wurden die Mahlzeiten bei Kerzenlicht serviert. Die bekannten Wahrzeichen der Stadt, das Opernhaus und die Hafenbrücke, die sonst während der Nacht im Flutlicht erstrahlen, blieben dunkel. 

Einer der Initiatoren der Aktion, Greg Bourne vom WWF-Australien, sagte, es habe Monate gedauert, die Aktion zu planen. Erhebliche logistische Probleme seien dabei zu bewältigen gewesen, um all die Verantwortlichen der großen Büro- und Geschäftszentren der Stadt unter einen Hut zu bringen. Die so genannte Earth Hour, die auch von Regierung von New South Wales unterstützt wurde, sollte ein Zeichen setzen, ein Zeichen für ein stärkeres Bewusstsein zur Problematik der globalen Erwärmung. Die Initiatoren hoffen, dass dieses Zeichen auch weltweit gehört wird. Den Australiern sollte der Gedanke nahe gebracht werden, dass jeder durch sein Verhalten dazu beitragen könne, den Ausstoß von Treibhausgasen zu verringern. Jeden Tag würden beispielsweise in Privathaushalten und Büros Lampen und Computer einfach eingeschaltet gelassen, ohne dass sie gebraucht würden. Allein durch das Abschalten nicht gebrauchter Stromverbraucher könne der Ausstoß von Treibhausgasen in der Stadt Sydney um fünf Prozent reduziert werden, klärten die Veranstalter die Bevölkerung der Stadt auf.

Australien gilt als der größte Pro-Kopf-Erzeuger von Kohlenstoffdioxid weltweit.