A strong earthquake has hit the city of Christchurch on New Zealand's South Island, killing scores of people.

A huge rescue effort is under way to save up to 200 people feared trapped beneath collapsed buildings. Prime Minister John Key said: "We may be witnessing New Zealand's darkest day."

A state of emergency has been declared in the city, where people could be seen wandering the rubble-strewn streets in shock.

The multi-storey Pyne Gould Guinness Building, where more than 200 people work, collapsed with dozens feared trapped inside.

Rescuers have been plucking casualties from the wreckage. Screams could be heard from those still trapped inside the Pyne Gould Guinness Building.

Buildings across the city have been extensively damaged because of the quake, which struck at lunchtime when the streets of Christchurch were filled with crowds.

Power and telephone lines were knocked out, and pipes burst, flooding the streets with water, and leaving some suburbs "unliveable".

The earthquake also dealt severe damage to the famous Christchurch Cathedral. Its spire toppled into the square below.

Victims included people crushed in vehicles hit by falling buildings.