As each time zone across the globe reaches the appointed hour, skylines are going dark and landmarks are dimming for Earth Hour, a global switch-off aimed at revitalising efforts against climate change.

At 8:30pm AEDT yesterday, Sydney's iconic Opera House and Harbour Bridge plunged into darkness, along with millions of homes.

Landmarks across Australia went dark, including Parliament House in Canberra and Melbourne's Luna Park and Federation Square.

Sydney Harbour ferry horns blared to signal the energy saving event, which is supported by 4,000 cities in a record 125 countries and includes 1,200 famous landmarks from the Forbidden City to the pyramids to the Las Vegas Strip.

And other iconic landmarks across the world are also dimming, from a Manila shopping centre to the Eiffel Tower in Paris.

Russian officials say they hope to beat last year's participation figure of six million people in 20 cities.

"From Brazil to America, to Canada, all the way down to Australia, Japan and India - it's a really diverse set of countries taking part this year," Earth Hour executive director Andy Ridley said.

Sydney's office buildings plunged into gloom at 8:30pm (AEDT) yesterday, setting off a rolling wave of darkness which swept the globe in a boost for the environmental movement after December's disappointing Copenhagen UN talks.

Greg Bourne from the environment group WWF says it is a symbolic event that boosts the awareness of how energy can be saved and greenhouse gases reduced.

"It might be a small symbol but by the time you get more than a billion people take part, it's a pretty big one," he said.

"But in the end we would obviously like Earth Hour to be every hour. We'd like people to be conscious of their impact on the planet.

"This isn't about reducing lifestyles, this is about changing the way we deliver those lifestyles."

The event had officially begun nearly three hours earlier when New Zealand's Chatham Islands switched off their diesel generators, leaving just 12 street lamps burning. It will eventually end in Samoa after nearly 24 hours.

Participation down

Electricity usage reports are indicating that fewer people and businesses took part in last night's Earth Hour in the Sydney CBD than in previous years.

Energy Australia recorded a 6.3 per cent reduction in energy use in the city during the event, equivalent to 15.9 tonnes of carbon emissions.

Paul Myors from Energy Australia says that is one of the smallest reductions in the event's history.

"This is the fourth Earth Hour event for Sydney and we've seen reductions vary between about 6 and 10 per cent over those four years," he said.

"So there's been no particular trend over that time, but this year's reduction wasn't as strong as last year's.

"We really just measure the impact on the electricity grid. But nonetheless I think it's a very good result, and I think the message of using energy efficiently to not only reduce greenhouse but our energy bills is still an important one."

In Europe, London's Big Ben and Manchester United's Old Trafford football ground were set to take part.

In America, some 30 states were on board, with Mount Rushmore, San Francisco's Golden Gate Bridge and Chicago's 110-storey Sears Tower all due to go dark.

But in Bangkok, city authorities were ordered to halt their Earth Hour campaign for security reasons as anti-government protesters held a major rally.

This year, users of Twitter and Facebook can show their support with special applications that turn their displays dark.

In December, two weeks of UN talks in Copenhagen failed to produce a binding commitment to limit global warming or set out concrete plans for doing so, in a setback for the environmental movement.

- ABC/AFP