A powerful earthquake killed at least 65 people in New Zealand’s second city Christchurch Tuesday, crushing buildings and vehicles and leaving hundreds trapped and screaming for help.

"We may be witnessing New Zealand’s darkest day," Prime Minister John Key said after the 6.3-magnitude quake pummelled the city, just six months after buildings were weakened by a 7.0 quake that miraculously claimed no victims.

"People are just sitting on the side of the road, their heads in their hands. This is a community that is absolutely in agony," Key said, warning that the toll was likely to rise.

New Zealand’s deadliest tremor in 80 years struck as city streets were packed with lunchtime shoppers, and turned central Christchurch into a rubble-strewn disaster zone littered with dazed and bleeding residents.

The city’s iconic cathedral lost its spire, and the six-storey Canterbury TV building was reduced to a smoking ruin.

Rescue helicopters plucked survivors to safety from the rooftops of buildings where staircases had collapsed, and emergency workers used giant cranes to pull office workers out of ruined city buildings.

"They are going to come and get you down. Just keep away from the edge," one woman yelled to a distraught colleague trapped on the top level of what had been a four-storey building, but which folded like a concertina.

The distressed woman was rescued by fire workers on a crane soon afterwards, and hugged her colleagues after reaching the ground which was littered with shattered glass, office paperwork and broken computers and desks.

Media reports quoted Christchurch’s mayor Bob Parker as saying that up to 200 people may still be trapped inside ruined buildings. Police drafted in urban search and rescue teams in an urgent attempt to locate survivors.

A woman trapped under her desk in one of the city’s worst-hit buildings gave an interview by telephone from beneath the rubble.

"I know I am bleeding and I can feel the ground is quite wet. I think it’s blood," Anne Voss told an Australian television station, adding that she believed one of her hands was injured.

Miranda Newbury was on the third floor of another city building when the quake hit, forcing her to make her way down through a darkened, crumbling staircase.

"I really thought my time was up. When I finally got outside, there was dust everywhere - it looked like a war zone. Very surreal," she said.

Other churches were partly destroyed in the tremor and the local newspaper’s offices were badly hit. Reports said survivors there were frantically texting relatives as they took shelter under their desks.

"The centre of the city bore the brunt (in September) but nothing like this time, where it’s been absolutely devastated," Key said, adding that the air force was mobilising Hercules transporter planes for the relief effort.

"What was a vibrant city a few hours ago now has been brought to its knees."

Local station TV3 said dead bodies had been pulled from a hostel and a bookshop, and that a tourist was crushed to death in a van. All flights in the country were briefly suspended after a Christchurch control tower was damaged.

Power was cut to thousands of residents, mobile phone networks were disrupted and road and rail transport was badly hit after the violent tremor, which tore gaping fissures in asphalt.

Water mains were also burst, unleashing a torrent of water that joined with heavy rains to inundate the suburb of Bexley and cause surface flooding in other areas of the city.

Video footage showed a landslide crushing a small building, while passersby fled for their lives from underneath a collapsing awning. Several strong aftershocks pummeled the stricken city.

"This is about as bad as it gets," said Parker, who declared a five-day state of emergency and said emergency crews would work through the night to reach the dozens of trapped people.

"What the picture will be in the morning, God only knows," he added.

New Zealand national cricket team, in India for the World Cup, reacted in horror to the grim news.

"Thoughts go out to the people of Chch today. Terrible thing to have happened again," batsman Martin Guptill wrote on Twitter.

The quake is the most deadly to hit New Zealand since a 7.8-magnitude tremor killed 256 people in the Hawke’s Bay region in 1931.

The September 4 tremor, measured at 7.0 magnitude, struck overnight and damaged 100,000 homes but claimed no lives.

Seismologists said that despite being smaller, the latest tremor was more destructive than the earlier quake because it was nearer to Christchurch’s centre and much closer to the earth’s surface.

New Zealand sits on the "Pacific Ring of Fire", a vast zone of seismic and volcanic activity stretching from Chile on one side to Japan and Indonesia on the other.