Live video footage showed parts of buildings collapsed into the streets, which were strewn with bricks and shattered concrete. Sidewalks and roads were cracked and split, and hundreds of dazed, screaming and crying residents wandered through the streets as sirens blared throughout the city.
Christchurch Mayor Bob Parker said he was on the top floor of the city council building when the quake hit, throwing him across the room.
"I got down onto the street and there were scenes of great confusion, a lot of very upset people," he said. "I know of people in our building who are injured and I've had some reports of serious injuries throughout the city."
Radio New Zealand reported that a church near the city center collapsed. The station also said staffers in its Christchurch newsroom had to cling to their desks during the shaking, with large filing cabinets toppling over.
Some cars apparently parked on the street were buried under rubble.
"What I can see from where I am in the central city is that there are significant amounts of additional damage," Parker said.
The U.S. Geological Survey said the temblor was centered 3 miles (5 kilometers) from the city at a depth of 2.5 miles (4 kilometers).
Christchurch has been hit by hundreds of aftershocks since a 7.1 magnitude earthquake struck Sept. 4 last year, causing extensive damage and a handful of injuries, but no deaths.
The city is home to about 350,000 people and is considered a tourist center and gateway to the South Island.
New Zealand sits on the Pacific "ring of fire" -- an arc of earthquake and volcanic zones stretching from Chile in South America through Alaska and down through the South Pacific. It records more than 14,000 earthquakes a year -- but only about 150 are felt by residents, and fewer than 10 a year do any damage.
The Sept. 4 quake wrecked hundreds of buildings in the city, and caused an estimated 4 billion New Zealand dollars ($3 billion) in damage. A strong aftershock in December caused further damage to buildings.
The city was still rebuilding from those quakes when Tuesday's temblor hit.