"The death toll I have at the moment is 65 and that may rise. So it is an absolute tragedy for this city, for New Zealand and for the people that we care so much about," New Zealand Prime Minister John Key said.
"It's a terrifying time for the people of Canterbury," he added.
"We may be witnessing New Zealand's darkest day," he said and described Christchurch - the nation's second-largest city - as "a scene of utter devastation."
Dozens were trapped under flattened buildings in the country's most devastating tremor in 80 years.
The widespread damage came from a lunch-time tremor in the city of about 4 lakh residents which is still recovering from a powerful quake in Sep last year. However, that quake did not cause any fatalities but did weaken many buildings.
Live video footage showed parts of buildings collapsed into the streets, which were strewn with bricks and shattered concrete.
Footpaths and roads were cracked and split, and hundreds of dazed, screaming and crying residents wandered through the streets as sirens blared throughout the city.
The TV footage showed onlookers clinging to each other and others bleeding and limping, many searching for loved ones and trying to reach trapped people.
People fled the city centre as emergency services rushed to the area to evacuate those trapped in buildings.
Local police said that there were multiple fatalities in the city, including an accident in which two buses had been crushed by falling buildings.
Christchurch airport was temporarily closed and police said they were evacuating the city centre as building frontages collapsed, with witnesses saying there were people trapped inside.
The city's iconic cathedral lost its spire and the six-storey Canterbury TV building was reduced to a smoking ruin due to the quake.
Eye-witnesses said rescue helicopters were pressed into service to lift 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.
"We had everything on our side in the last earthquake ...we're the polar opposite this time," the Prime Minister said.