New Zealand's prime minister says at least 65 people have died after a 6.3-magnitude earthquake hit Christchurch.
John Key said the toll was expected to rise further, adding: "We may be witnessing New Zealand's darkest day."
The tremor struck at 1251 (2351 GMT on Monday), only 10km (6.2 miles) south-east of the city and at a depth of 5km (3.1 miles), causing widespread damage.
Mr Key said the emergency services were working as fast as they could to find people trapped under the rubble.
The damage is said to be far worse than after the 7.1-magnitude quake on 4 September, which left two people seriously injured but no fatalities.
Its epicentre was further away from the city and deeper underground, but still caused an estimated $3bn (£1.9bn) in damage.
Community 'in agony'
TV pictures of the aftermath of Tuesday's earthquake showed scores of collapsed buildings in Christchurch, on South Island.
People could be seen wandering the rubble-filled streets in distress.
Local police said there were reports of multiple fatalities, including people on two buses which had been crushed by falling buildings.
There were also reports of fires and of numerous people being trapped in collapsed buildings, they added. Officials said up to 30 people were feared still trapped inside the Pyne Gould Guinness building.
Local television showed bodies being pulled out of rubble strewn around the city centre.
Christchurch Cathedral, an iconic stone building in the centre of the city, was partly destroyed, its spire toppling into the square below.
"I was in the square right outside the cathedral - the whole front has fallen down and there were people running from there - there were people inside as well," John Gurr, a camera technician, told the Reuters news agency. "Colombo Street, the main street, is just a mess."
Radio New Zealand reported that its staff had to cling to their desks during the tremor, and that a church near its offices had collapsed.
Concrete in Victoria Square had risen at least a metre in some places and there are signs of liquefaction around the Avon river, RNZ added.
Power and telephone lines were knocked out, and pipes burst, flooding the streets with water.
The suburbs of Lyttleton and New Brighton are reportedly "unliveable".
Grant Beedie, who was working in a factory in Sydenham, told the BBC that the damage was "massive… like a warzone".
"The entire building just shook. We all just hit the ground and shut our eyes. There were loads of things flying off the shelves," he said. "There were massive machines moving about - it was scary. I was almost in tears."
Christchurch mayor Bob Parker said there was "a lot of devastation".
"We're really in a position where we are now just trying to build up a picture of the scale of this and areas of damage and trying to quantify it in some way," he told the BBC.
"We're beginning to get a feeling, from what we've got so far, that a significant number of people are injured and a significant number of people are still trapped in buildings," the mayor added.
"And as we get further into that rescue operation... clearly we're going to be presented with statistics that are going to be bleak."
Just how bleak was soon revealed by the prime minister, who said 65 people were confirmed dead and scores more were injured or missing.
"It is just a scene of utter devastation," Mr Key said. "We have to work as fast as we can to get people out of environments where they are trapped."
"This is a community that is absolutely in agony. We just need this community, as it has done before and as it will do again, to come together, to check and support one another," he added. "We will get through this. New Zealand will regroup and Christchurch will regroup."
The military has been called in to assist the rescue effort, and the government has accepted the offer of help from specialist from Australia.
Civil Defence Controller David Coetzee said "significant" aftershocks should be expected. Some have been as powerful as magnitude 5.
New Zealand lies at the southern end of the so-called Pacific Ring of Fire, and above an area of the Earth's crust where the Pacific Plate converges with the Indo-Australian Plate.
The country 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.