"This is a community that is absolutely in agony," Key said. "We will get through this. New Zealand will regroup and Christchurch will regroup."
Click here to see photos of the earthquake's aftermath.
The 6.3 magnitude quake struck at 12:51 p.m. Tuesday local time, and was very shallow in depth, measuring 2.5 miles beneath the surface, causing violent shaking, and widespread damage.
"The building just exploded," resident Barry Saunders said in an interview with Radio New Zealand. "It was just like a movie. It took three or four seconds to comprehend what was going on."
The U.S. Geological Survey said today's quake was centered three miles from the city.
This latest tremor came five months after a 7.1 magnitude earthquake struck the city of 350,000.
That quake in September caused no deaths but badly damaged buildings throughout the city, many of them the same structures that collapsed today.
Christchurch Mayor Bob Parker declared a state of emergency shortly after the quake hit, and ordered people to evacuate the city center. He said it was impossible to say how many people are still trapped in the rubble, but that it is estimated to be more than 100.
Troops were deployed to help victims trapped amid the rubble, the airport was closed, and the Christchurch Hospital briefly evacuated before it was deemed safe for patients to return.
Some people were stuck in office towers, forcing firefighters to extend their ladders to rescue people trapped on roofs. Among the dead were passengers aboard two buses crushed by falling buildings.
New Zealand television showed residents walking amid the rubble in a daze, some badly injured from falling debris.
Newspaper editor Andrew Holden said he heard glass cracking and falling throughout the building, and clouds of dust billowing through the newsroom.
"The roof above the main staircase collapsed so there was a fair bit of rubble down the staircase," Holden said in an interview with Radio New Zealand. "We've had some good size shocks before but when you've got a quake of that size that just continues on and on."
ABC News Radio and The Associated Press contributed to this report.