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Prime Minister John Key has told reporters that the death toll from today's magnitude 6.3 earthquake stands at 65, and Christchurch Mayor Bob Parker said it could quite possibly double.
Speaking from Christchurch after flying down this afternoon, Mr Key told One News it was likely the quake would prove to be New Zealand's "darkest day".
TVNZ reports up to 200 people remained trapped in damaged buildings.
Mr Key said New Zealanders should be assured the Government was doing everything it could.
There were 180 police on the ground, a further 200 were on the way and 350 military were already in Christchurch with another 250 to arrive soon.
The Government had accepted offers of specialist help from Australia and US.
"I don't think we can go past the fact that we may well be witnessing New Zealand's darkest day."
The latest "very fluid" advice was that the death toll 65, "and that may rise".
"It's an absolute tragedy for this city, for New Zealand, for the people we care so much about and it's a terrifying time for the people of Canterbury.
"Quite frankly it's hard to know what to say.
"It's just a scene of utter devestation, it's just so vastly different from the last earthquake where, yes, there were some horrifying scenes but this is central city at a very, very busy time where you've had a massive earthquake.
"It's been violent shaking on probably what was very weakened infrastructure."
Christchurch mayor Bob Parker says: "As daylight comes tomorrow we need to prepare ourselves for what is likely to be significant numbers of casulaties."
Search teams to work through the night
Search and rescue teams are to work throughout the night looking for survivors following today's massive Christchurch earthquake, Civil Defence director John Hamilton says.
"We have to be prepared to accept that it is going to be a heavy toll," Mr Hamilton said.
He could not say how many people were trapped in buildings, saying the information was "patchy".
"There could well be people who are stuck in buildings overnight. I can't confirm, but I would expect that's in all probability the case."
Mr Hamilton said an urban search and rescue team from Australia was on its way, and was expected to arrive after midnight.
A team has also been deployed from Auckland and a second team would be sent down as well.
Mr Hamilton said ambulances from other areas in the South Island were also on its way to Christchurch hospitals and other centres.
He said a large welfare centre has been established in Hagley Park, which would provide shelter for people who can't go home, but it is now full.
"I would expect the civil defence authorities in Christchurch to open welfare centres across the city to take in and support people whose homes are unavailable."
Superintendent Dave Cliff, Canterbury Police District Commander, said: "This is a sad day for Christchurch, and indeed New Zealand. Our hearts go out to all those who are suffering from today's awful events."
Emergency services are continuing to search the central city, particularly the high rise buildings in the central business district, many of which have been extensively damaged.
The central city business district has been cordoned off and police are warning people to stay away from the areas within Madras, St Asaph, Montreal and Kilmore Streets.
People who try to get into the cordoned off areas will be turned away.
Police are urging people to:
- stay out of the central city
- check the welfare of your neighbours, especially the elderly
- conserve water
Hospitals making way for injured
Hospitals in other South Island cities and towns were discharging patients to make room for injured quake survivors and out-of-town ambulances were reported flooding into Christchurch to help harried local medical teams.
Rescue squads were also heading for the stricken city in the face of reports of people trapped in crumpled buildings, some of them high-rise.
The Pyne Gould Guinness building, several storeys high, folded up like a pack of cards and rescuers were still trying to find trapped occupants tonight.
Up to 50 people were said to be in the wreckage - alive or dead.
Rescuers were trying to get people out of the Canterbury TV building in Madras St, while firefighters battled a fire there.
The earthquake shook the city at 12.51pm while streets were crowed with shoppers and office workers.
It was not as powerful as the 7.3 that struck in early morning hours of 4 September last year but was much shallower, leading to greater damage.
Scientists put the epicentre at 10km southeast of the city - apparently in the middle of the harbour at Lyttelton, the city's coastal port - at a depth of only 5km.
Radio and television reported damage in the town of Lyttelton was severe.
The road tunnel linking Christchurch and Lyttelton was closed and Christchurch International Airport was shut down.
One runway was expected to be cleared late this afternoon and Prime Minister John Key was expected to fly in from Wellington.
Christchurch has suffered a long series of after shocks following last year's quake and they continued after the big one today.
One tremor of 5.7 occurred five minutes after the main shock and there was another of 5.5 at 2.50pm.
Late in the afternoon after shocks were still occurring in the magnitude 4 range on the Richter Scale.
The city's iconic cathedral in the central square survived September without little damage but its spire fell today and the rest of the building was badly damaged.
Christchurch mayor Bob Parker said the damage was much more severe than in the great 7.1 quake almost six months ago.
"The city centre is like a war zone and damage is immense.
"Everybody needs to understand that this is going to be a day of very black news," the mayor said.
He declared a state of emergency, adding Christchurch and Canterbury would need help from the rest of the country.
The quake ravaged the city centre as display widows shattered and walls and roofs fell.
Defence Force troops and extra police have been drafted in from other parts of the country to aid in rescue efforts.
One third of the city was reported to be without electricity and Contact Energy closed its LPG gas lines after reported of leaking gas.
The telephone system was severely overloaded as people called to check on the safety of friends and relatives.
The central city is being evacuated as police receive reports of widespread damage.
Bodies lying in streets
Earlier today, residents reported were bodies lying in the streets of Christchurch.
Police said fatalities had been reported at several locations and that two buses had been crushed by falling buildings. Christchurch mayor Bob Parker has declared a state of emergency.
Christchurch resident Jaydn Katene told the Herald: "We've had friends in town call us and say there are just bodies lying around; lots of dead bodies outside shops just lying there just covered in bricks.
"When it hit we were knocked to our feet. Everything in the house fell down, nothing was left still standing. There's more damage than the first earthquake, the roads are completely torn up; sewage coming up and flooding. It's crazy."
"The elderly are all crying. The next-door neighbours around us were all bawling their eyes out, it was horrible. People can't get out of their houses," said Mr Katene.
"We've seen cars halfway sunken into the road. We've heard there's a bus which is sunken halfway into the road just around the corner.
"Buildings are half-collapsed everywhere.
"It smells horrible. The roads are packed with cars. There aren't enough police or ambulances. Houses are all collapsing. It's pretty shocking; a total warzone."
'Grave concerns' for Banks Peninsula
Christchurch Mayor Bob Parker says he holds grave concerns for the safety of people in Banks Peninsula area following today's massive earthquake and aftershocks.
Christchurch is located immediately north of the peninsula.
"We still have yet to hear any reports in from Banks Peninsula and I'm very very concerned about that," he told Radio New Zealand.
"The centre of the earthquake was in Lyttelton Harbour. There are hills covered in rocks, those rocks would have come down."
Mr Parker said the damage to the city centre was immense and people were trapped in buildings.
"Everybody needs to understand that this is going to be a day of very black news," he said.
Canterbury would need help from the rest of the country.
"There is no doubt that we are going to have a significant number of homeless people in our city," he said.
The region had lost a lot of communication, and water was coming up through the streets from pipes.
He urged people to stay calm despite the extremely difficult circumstances.
Mr Parker said people were very frightened but they needed to keep the roads clear.
"Don't use the phones unless you have to, those phones are also our lifeline."
When the quake struck the mayor was knocked over.
"I looked out over the city once I got up and I could see clouds of dust from buildings collapsing. I could hear screams from streets."
- NZ Herald staff