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Rescuers will work throughout the night in miserable weather looking for survivors following a 6.3 earthquake which claimed at least 65 lives, Civil Defence director John Hamilton said.
Emergency services were focussing on high rise buildings in the central business district, many of which were extensively damaged, police said.
The Pyne Gould Guinness building has tilted at an awkward angle and slumped to the ground with 30 people thought to be inside, while people are trapped under desks in the Christchurch Press building opposite Christ Church Cathedral.
Fatalities have been reported in the Canterbury TV building, while the Forsyth Barr Tower has lost its stairs, so those trapped high above ground had to be lifted out by crane.
People are also feared trapped in hotels, Civil Defence Minister John Carter said.
"What we don't know is whether they were out looking around the town or were in their rooms.''
Police said the immediate focus was rescuing people trapped in buildings following the 12.51pm quake.
Around 200 extra staff - some victim identification specialists - were being sent from around the country to Christchurch to help in the rescue effort.
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.
Rescue teams from New Plymouth, Auckland and Australia are also on their way to the disaster zone, Mr Carter said.
A woman was rescued from the roof of Pyne Gould Guinness building, where 200 people were working when the quake struck.
Family today gathered outside in hope their loved ones had escaped with their lives.
The Forsyth Barr building on the corner of Armargh and Colombo streets was also heavily damaged, with an estimated 150 inside.
About 30 people initially escaped the 13-storey building, the Fire Service said.
TV3 said the provincial chambers building had collapsed and people were believed to be trapped inside.
Two people remain trapped in the elegant The Press building on Cathedral Square, after the top three floors of the four-storey building collapsed.
Press reporter Martin van Beynen said the mood was still hopeful among families gathered outside the building.
"People are teary eyed and hugging each other,'' he said.
A young father-of-three - with another baby on the way - said he thought of his loved ones as he awaited rescue in the Canterbury TV (CTV) building after the quake flattened it.
Tupi Emery was trapped for five hours while his mother, Tania, stood in vigil outside the central city building, reduced from a seven-storey building to a burning pile of rubble.
He had gone to see a doctor on the fourth floor of the building, and his mother had just left when the quake struck.
He texted her continually while he was trapped.
"Really, I thought he was gone,'' his mother said.
Mr Emery said he could hear other trapped people calling out for help.
"I was banging on the metal, yelling out 'help', because my voice was better than theirs,'' he told TVNZ's CloseUp.
He suffered cuts and burns and when asked how he was feeling, he replied: "Alive.''
He said he now planned to "live life to the fullest''.