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(CNN) -- A 6.3-magnitude earthquake ripped through Christchurch, New Zealand, on Tuesday afternoon, causing multiple fatalities as it toppled buildings onto buses, buckled streets and damaged cathedrals, authorities said.

New Zealand Police announced on the agency's website that a large-scale evacuation of the central city was under way. According to the news release, the earthquake killed an undetermined number of people at various locations around the city, including passengers on two buses crushed by buildings that had fallen on them.

TVNZ reported that the 147-year-old Christchurch Cathedral's spire had toppled, Christchurch Hospital was being evacuated and the airport was closed.

Laura Campbell told CNN she was at work at the bottom of a six-story building when the earthquake struck. She described seeing "windows blowing out, bricks falling down, people screaming, the whole nine yards."

"It was bloody serious," said Campbell, who was trying to walk home. "I'm worried about what I'm going to find down the road."

The U.S. Geological Survey said the quake struck just before 1 p.m. Tuesday (7 p.m. Monday ET), and a 5.6-magnitude aftershock struck about 15 minutes later. The U.S.G.S. recorded a second 5.5-magnitude aftershock shortly before 3 p.m. local time.

The New Zealand Herald reported that phone lines in the area were out, including the city's emergency 111 service, roads were cracked -- in some cases lifted as much as a meter (1 yard) -- and water mains had burst, flooding several streets.

Witness Philip Gregan said he was attending a joint U.S.- New Zealand conference when the earthquake struck.

"I'm seeing a lot of damage in buildings, there are glass and bricks in the road. I've seen one collapsed bridge and there's a lot of water from broken water mains," he told CNN. "I saw one (injured) person in the back of a police car and one of our colleagues saw a person crushed by falling debris, so there are definitely dead."

New Zealand's transit authority told TVNZ that it had been unable to reach its staff in Christchurch and at the Lyttleton Tunnel, which is near the epicenter.

Christchurch police told TVNZ that the city's 106-year-old Cathedral of the Blessed Sacrament was badly damaged, and a Herald reporter said that half the building had collapsed.

Camera footage aired by the station showed piles of stone lying atop crushed chairs on the floor of the cathedral with light shining through the collapsed tower above the sanctuary.

Witness Gavin Blowman told CNN how he ran into the street when the earthquake struck.

"It felt like I was running on jelly," he said. "We saw a giant rock tumble to the ground from a cliff -- a rock that had been there for millennia. It fell on the RSA (Returned Services Association, a veterans' association) building -- it was terrifying."

He said there were now fears that a tsunami could hit in the wake of the quake and that he and 20 other people were trying to get to higher ground.

Flying out of the city currently was not an option in the hours immediately after the quake.

"The airport is currently closed until further notice," an airport employee told CNN some two hours after the quake.

All planes inbound for the airport were being diverted, and no planes were allowed to take off. No one was injured at the airport, he said. The employee did not give his name and wanted to clear the phone line to continue emergency work.

The quake caused significant damage to several older buildings, a Herald reporter said.

New Zealand Prime Minister John Key said he would attend an emergency Cabinet meeting in Wellington and then fly to Christchurch if conditions allow.

Christchurch Mayor Bob Parker told Radio New Zealand that the rumbling tossed him across the room, that he knew of injuries in the city council building and has heard unconfirmed reports of serious injuries.

"That was, in the city central anyway, as violent as the one that happened on the 4th of September," he said.

Parker added that streets were jammed as people tried to get out of the city, and he urged people to avoid the water supply.

"We've been through this before this once, we now need to think what we did at that time," he said.

Southern New Zealand has been plagued by a series of quakes since September, when the area was shaken by a 7.1-magnitude temblor that New Zealand authorities said was the most damaging quake to hit the region since 1931. The earthquake struck in the predawn hours of September 4. Authorities said the deserted streets at that time likely kept injuries to a minimum.

There were no deaths from that earthquake.

Both the Roman Catholic Cathedral of the Blessed Sacrament and the Anglican Christchurch Cathedral were undergoing repairs after being badly damaged by the September quake.

The quake struck in the "worst possible location," Kevin Fenaughty, data center manager for GNS Science, told the Herald.

"It's a nightmare," he said. "A lot of people were just getting back on their feet after the original quake."

TVNZ also reported that its newsroom in Christchurch was badly damaged.