A strong, 6.3 magnitude earthquake has rocked the southern New Zealand city of Christchurch, killing at least 65 people.

New Zealand Prime Minister John Key told reporters in the city that the death toll was expected to rise further.

"It is a just a scene of utter devastation," Key told TV One News. "This may be New Zealand's darkest day."

Earlier, a police statement said "multiple fatalities have been reported at several locations in the central city, including two buses crushed by falling buildings."

"Other reports include multiple building collapses, fires in buildings in the central (city) and persons reported trapped in buildings," the statement said.

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People rush past damaged buildings in Christchurch, New Zealand, Tuesday after a strong earthquake rocked the city. ((TV3/Associated Press))

Christchurch's daily newspaper the Press reported on its website that witnesses said screams could be heard coming from the Pyne Gould Business building, where as many as 30 people were feared trapped.

Christchurch Mayor Bob Parker said between 150 and 200 people were believed trapped in buildings across the city. Parker had declared a state of emergency.

The spire of downtown's Christchurch Cathedral  collapsed into a central city square.

"The top of the cathedral's collapsed," a police official told Sky News in the city's centre. "It's the heart of the city. It's gone."

Video footage showed some multi-storey buildings collapsed in on themselves, and others with walls that had collapsed into the streets, strewn with bricks and shattered concrete.

Sidewalks and roads were cracked and split, and thousands of dazed, screaming and crying residents wandered through the streets as sirens blared.

Groups of people helped victims clutching bleedings wounds, and others were carried to private vehicles in makeshift stretchers fashioned from rugs or bits of debris.

The quake first hit at 12.51 p.m. local time Tuesday, according to New Zealand's GNS Science. It was centred at Lyttelton, N.Z., southeast of Christchurch, at a depth of five kilometres.

Aftershocks were continuing, the New Zealand Herald reported. They included a 5.7 magnitude aftershock at 1.04 p.m. at a depth of six kilometres, 10 kilometres south of Christchurch.

The city's airport was closed after the flight tower collapsed, and a major tunnel was shut. Christchurch Hospital remained open, but damaged, the Press reported.

Power was believed to be out in 80 per cent of the city.

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Civil authorities set up "triage centres" at several locations around the city to help injured people.

People working in the centre of Christchurch interviewed by Sky News New Zealand said the quake was much worse than last year's.

Because the quake occurred at lunchtime on a busy Tuesday, many more people were hurt. The Sept. 4, 2010 earthquake occurred on a weekend.

Radio New Zealand said staffers in its Christchurch newsroom had to cling to their desks during the shaking, with large filing cabinets toppling over.

Christchurch has been hit by hundreds of aftershocks since a 7.1 magnitude earthquake struck Sept. 4, causing extensive damage and a handful of injuries, but no deaths.

The U.S. Geological Survey initially said the temblor was centred five kilometres from the city at a depth of four kilometres.

Reuters reported that the country, located between the Pacific and Indo-Australian tectonic plates, records an average of more than 14,000 quakes a year. Only 20 would normally top magnitude 5.0, the news agency said.

With files from The Associated Press