Prime Minister John Key announced tonight that "at least 65 people have lost their lives'' and noted the rescuers were still scrabbling through the ruins of collapsed buildings looking for injured and trapped survivors - and bodies.
Christchurch Mayor Bob Parker tonight said more than 100 people were trapped in about six sites - but it was possible more were trapped in individual houses.
Police have launched a hotline, 0800 779997, that people can call to report missing people.
The death toll is already the second highest in a New Zealand earthquake - outranked only by the 256 people who died in the violent 7.9 1931 Hawke's Bay quake, whose 70th anniversary was marked earlier this month.
Police have reported "multiple fatalities'' at several locations in the downtown area, including where two buses were crushed by falling buildings.
Today's shake, measured at magnitude 6.3, followed the massive 7.1 tremor last September 4. That one struck in the early hours of the morning and no one was killed.
Today's earthquake jolted the city at 12.51pm, the worst possible time with the central city packed with lunch-hour shoppers, office workers and many school children.
Victims were crushed to death as buildings collapsed, many of them weakened in last year's event.
The quake was not as powerful as the September 4 shake 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 Pyne Gould Corp 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 famous cathedral in the city's downtown square which stood undamaged last September lost its spire today and suffered heavy damage.
Reports that people were in the spire when it tumbled into the church itself and the square outside could not be verified immediately.
Police superintendent Dave Cliff said one police staff member was unaccounted for.
"We are doing everything possible to free people who are trapped,'' he said.
Mr Key flew to Christchurch this afternoon and after a quick tour of the city described it as "utterly wrecked", adding "this is an absolute tragedy for Christchurch''.
"We may well be witnessing New Zealand's darkest day,'' he said.
Graphic photographs on television showed blood-splattered survivors scrambling from downed buildings or crawling from under shattered shop verandahs that had fallen on to city footpaths.
Crushed cars lined parking spaces, masonry scattered across roads.
Mr Parker, lauded for his behaviour and slick civic control in the wake of last year's quake, said the damage today was much more severe.
"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,'' he said.
He declared a state of emergency, adding Christchurch and Canterbury would need help from the rest of the country.
Christchurch Airport closed after today's earthquake and was to remain shut down overnight, open only to emergency flights and aircraft carrying rescuers and medical helpers.
Extra police, armed forces troops and search and rescue teams were heading for Christchurch tonight and the inter-island ferry heading for the South island from Wellington carried a group of 40 police, among them victim-identification specialists.
A 73-strong New South Wales search and rescue team arrived in Christchurch tonight and will go to work at first light tomorrow.
The road tunnel under the Port Hills linking Christchurch and Lyttelton was closed for some hours but was reopened tonight for emergency vehicles.
The earthquake ruptured sewer and water lines, still under repair after last September, and LPG gas lines were shut off after consumers reported leaking gas.
One third of the city was without electricity tonight after the quake brought down lines and poles.
Mr Parker urged people to put out pots and pans to collect rain water tonight, and said they should not shower, bath or use the toilet.
People should also stay off the roads and stay well away from the central city.
Heavy aftershocks continued to roll across Christchurch tonight, further frightening shocked residents. A 5.0 magnitude jolt hit at 7.43pm.
Some reports said most damage was confined to Christchurch and nearby areas and that rural property had not suffered greatly.
But on the West Coast, the face of the immense Tasman Glacier collapsed in enormous chunks into its lake following the earthquake.
