The two aftershocks -- one magnitude 5.6 and another 5.5 -- shook Christchurch within hours of the initial 6.3 earthquake.
The aftermath caused buildings to crumble, including the 130-year-old Christchurch Cathedral. Its spire toppled onto the city square. Afteshocks continued through the night, some with a magnitude of 5.
"To see the top of the cathedral come down, I've never seen anything like it," one survivor said. "There's just no words to be quite honest. It was just absolutely awful."
Phone service has been crippled, and 80 percent of the city has no electricity or water.
"This is much, much more serious than last time," said Roger Sutton, CEO of the power company Orion New Zealand, according to the New Zealand Herald. "We have some major power assets that have been damaged by this quake."
People have been urged to conserve water; not to shower, take baths or flush toilets. Temporary relief shelters have been set up. One in nearby Hagley Park was full and has turned people away. More than 2,500 people have shown up at the Ellerslie Flower Show, a scheduled event in Christchurch.
The tents for the show were being used as an overnight shelter for people. The show's organizers provided 1,500 meals, according to New Zealand's News Channel 3.
Google launched a person-finder application on its website to aid those looking for missing people. The website allows anyone to enter information about someone's whereabouts, which can then be viewed by the public. Similar sites were launched during the disasters in Haiti and Chile.
Houses have been devoured by mud, and the sides of buildings have been sheared off. Buildings have collapsed and bricks have scattered onto the streets. Roads and sidewalks have buckled, and water has gushed through giant cracks in the pavement.
Deputy Prime Minister Bill English reported that 350 military personnel were in Christchurch providing first aid, security and search and rescue services, and 250 others were preparing to relieve them. More military, police and search and rescue teams were expected to arrive overnight.
On the ground, chaos abounded. Residents were crying and screaming, and more than 100 people, including a dozen Japanese students, were still thought to be trapped in the rubble.
"There are about 10 people from my work in there," one woman said, standing outside her damaged office building.
Rescue workers battled debris and flooding to free those trapped in an office building in the city center.
"I was buried under part of the ceiling," one woman said.
"I think if we'd been at the end of the building. We would have all been squashed," another survivor said.
One man was lowered by rope to safety from a high-rise building.
"Stairwells, completely gone, 17 stories with just both stairs gone," another survivor said.
With such extensive damage, Prime Minister Key reassured residents that help from the United States and Australia was on the way.
"It's an absolute tragedy for this city, for New Zealand, for the people we care so much about," Key said.
"We may well be witnessing New Zealand's darkest day," he said.
Key described a scene of "utter devastation" and vowed to work as fast as possible.
"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.