Large parts of Sydney were plunged into chaos yesterday when electricity cables supplying power to Australia's largest city went down during the evening rush hour, Energy Australia said.

Emergency services authorities said many people were caught in lifts when the power went out for more than two hours around 5 p.m. (0600 GMT), blacking out office blocks and traffic lights and causing traffic snarl-ups.

The Sydney Harbour Tunnel was closed in both directions, along with major highways feeding into the city, with the Roads and Traffic Authority reporting heavy congestion and urging motorists to avoid the area.

The blackout also hit the famous Sydney Opera House, which was forced to cancel yesterday night's performances.

"The majority of the CBD (central business district) has been affected," New South Wales fire department superintendent Craig Brierley said.

"We've been inundated by calls from automatic fire alarms but also there are people caught in lifts. We don't know how many yet, but it seems like quite a few."

Describing the situation in the city, he said: "It's been a bit chaotic".

Energy Australia said about 70,000 homes and businesses had been affected, including swathes of central Sydney and the city's posh eastern suburbs.

The power company said there was an unspecified problem with four large cables supplying two sub-stations in the city centre.

"These four large cables have detected a problem and automatically shut themselves down - it's an automatic protection mechanism for safety reasons," Energy Australia spokeswoman Kylie Yates told ABC radio.

"Our crews are now investigating a whole range of options and doing tests to pinpoint the exact nature of the problem."

A police mouthpiece said power was restored shortly after 7 p.m. and it was believed office workers were no longer stuck in lifts. She was unable to say how many people had been trapped.

There were no reports of injuries.