Rescuers are toiling overnight in New Zealand to reach scores of trapped people after a 6.3-magnitude earthquake which has claimed at least 65 lives.
Christchurch Mayor Bob Parker says more than 100 people are feared buried in collapsed buildings in the city.
The disaster struck at a shallow depth of 5km (3.1 miles) on Tuesday lunchtime when Christchurch was at its busiest.
It is the South Island city's second tremor in six months, and the country's worst natural disaster in 80 years.
'Time is of essence'
The mayor has declared a state of emergency and ordered the city centre's evacuation.
On a cold and wet night, emergency teams have been toiling under floodlights to reach survivors, as relatives keep vigil outside.
Rescue teams with sniffer dogs have been fanning out across Christchurch.
Deputy Prime Minister Bill English said that 350 military personnel had been deployed to help with search and rescue efforts.
"The government is willing to throw everything it can in the rescue effort. Time is going to be of essence," Mr English said.
The government has accepted an offer of specialist help from Australia, whose rescuers are due to arrive in New Zealand shortly.
A series of aftershocks, some as big as magnitude 5, have rattled the stricken city of nearly 400,000 people.
Many power and telephone lines are knocked out, while burst water mains have deluged whole districts.
Up to 30 people were feared trapped inside the flattened Pyne Gould Guinness building, where screams have been heard from the ruins.
Trapped under her office desk, Anne Voss told a New Zealand TV station: "I rang my kids to say goodbye. It was absolutely horrible.
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Streets in the north-eastern suburb of Bexley were flooded as the quake caused water mains to burst, which coincided with heavy rain.
Pyne Gould Guinness
The multi-storey Pyne Gould Guinness Building, which normally houses around 200 workers, collapsed. A number of people were thought to be trapped inside.
The 63m spire of the city's Anglican cathedral was toppled by the earthquake. A New Zealand TV reporter took a look inside the damaged building.
Part of Christchurch's Canterbury Television [CTV] building completely collapsed in the earthquake. Some 24 people have been rescued from the building, but police said there might be between 60 and 120 bodies trapped underneath.
In the aftermath of the earthquake, Rhys Taylor took this video on Oxford Terrace, 50 metres away from the city's main hospital. He said: "Cars were being used as ambulances to transport the injured."
"My daughter was crying and I was crying because I honestly thought that was it. You know, you want to tell them you love them, don't you?"
She said she could hear other people alive in the building, and had called out to them.
The city's cathedral lost its spire, while a six-storey TV building housing an English-language school was reduced to a smoking ruin.
A dozen Japanese students at the school have been reported missing.
Emergency shelters have been set up at the city's Hagley Park, a race course, schools and community halls.
The Red Cross has been trying to find accommodation for people sheltering outside in tents or under plastic sheeting.
All the schools in Christchurch are closed until further notice, as expert teams are assessing any potential damage to the buildings.
New Zealand Prime Minister John Key, who held an emergency cabinet meeting before heading to the disaster zone, said: "We paid a very heavy price here.
"We could be looking down the barrel at New Zealand's darkest day."
In the aftermath of the disaster, shocked survivors wandered streets strewn with debris, including shattered glass, broken computers and desks.
Roads split and cracked open as the ground beneath was liquefied by the quake.
Helicopters plucked survivors to safety from rooftops, and dumped water on fires.
Bystanders used bare hands to try to free survivors trapped under debris.
Many injured people were carried out on blood-soaked stretchers or in the arms of shocked workmates and strangers.
Some escaped on ropes lowered from office towers. Others managed to crawl out of the rubble.
One Christchurch resident, Jaydn Katene, told the New Zealand Herald: "We've had friends in town call us and say there are lots of dead bodies outside shops just lying there just covered in bricks."
Police said that the dead included people on two buses which were crushed by falling buildings.
John Gurr, a camera technician, told Reuters news agency the area was "like a war zone".
The quake caused some 30 million tonnes of ice to shear away from New Zealand's biggest glacier.
Witnesses say massive icebergs formed when the Tasman Glacier in Aoraki/Mount Cook National Park broke, creating huge waves.
Britain's Queen Elizabeth II, who is also New Zealand's head of state, said in a statement she had been "utterly shocked" by the news.
"My thoughts are with all those who have been affected by this dreadful event," the statement said.
The damage is thought to be far worse than after the 7.1-magnitude quake on 4 September, which left two people seriously injured but no fatalities.
The epicentre of that quake, which occurred in the middle of the night, was further away from the city and deeper underground.
New Zealand experiences more than 14,000 earthquakes a year, of which only around 20 have a magnitude in excess of 5.0.
The last fatal earthquake was in 1968, when a 7.1-magnitude tremor killed three people on the South Island's western coast.
Tuesday's was the country's worst natural disaster since a 1931 quake in Hawke's Bay on the North Island killed 256 people.