WELLINGTON, New Zealand — A powerful 7.1-magnitude earthquake damaged buildings, cut power and knocked fleeing residents off their feet on New Zealand's South Island early Saturday, but there were so far no deaths and only two injuries reported.

Panicked residents in their pyjamas ran into the streets of the southern city of Christchurch after the pre-dawn quake, residents said. There were reports of some people trapped in damaged buildings -- though none appeared to be crushed by rubble -- and a few looters broke into some of the damaged shops in the city of 400,000, authorities said.

A state of emergency was declared and army troops were on standby to assist after the quake, which hit 19 miles (30 kilometres) west of Christchurch, according to the state geological agency GNS Science. No tsunami alert was issued.

Roads had been blocked by rubble, power and traffic lights were out, and gas and water supplies disrupted, while chimneys and walls had fallen from older buildings, Christchurch Mayor Bob Parker said. He warned that continuing aftershocks could cause masonry to fall from damaged buildings.

Suburban dweller Mark O'Connell said his house was full of smashed glass, food tossed from shelves, with sets of drawers, TVs and computers tipped over.

"We were thrown from wall to wall as we tried to escape down the stairs to get to safety," he told The Associated Press.

GNS Science initially reported the quake as magnitude 7.4, but later downgraded it after re-examining quake records. The U.S. Geological Survey, in America, measured the quake at 7.0.

Minister of Civil Defence John Carter stressed the low number of casualties.

"I think we've been extremely lucky as a nation that there's been no fatalities," Carter told reporters.

Still, infrastructure damage was major, with "a lot of damage to our key infrastructure ... water, waste water (sewerage) systems." Earthquake and insurance specialists would give an initial damage assessment within 48 hours, he said.

Experts said the low levels of injury reflect the strict building codes that apply in New Zealand, which records more than 14,000 earthquakes a year.

"New Zealand has very good building codes ... (that) mean the buildings are strong compared with, say, Haiti," which suffered widespread death and devastation in a magnitude 7.0 quake this year, earth sciences professor Martha Savage said.

"It's about the same size (quake) as Haiti, but the damage is so much less. Though chimneys and some older facades came down, the structures are well built," said Savage, a professor from the School of Geography, Environment and Earth Sciences at Victoria University in the capital, Wellington.

Christchurch fire service spokesman Mike Bowden said a number of people had been trapped in buildings by fallen chimneys and blocked entrances, but there were no reports of people pinned under rubble. Rescue teams were out checking premises.

Christchurch Hospital said it had treated two men with serious injuries and a number of people with minor injuries.

One man was hit by a falling chimney and was in serious condition in intensive care, while a second was badly cut by glass, hospital spokeswoman Michele Hider said.

Christchurch police reported road damage in parts of the city, with a series of sharp aftershocks rocking the area. Police officers cordoned off some streets where rubble was strewn about. Video showed parked cars crushed by heaps of fallen bricks, and buckled roads.

"There is considerable damage in the central city and we've also had reports of looting, just shop windows broken and easy picking of displays," Police Inspector Mike Coleman told New Zealand's National Radio.

Police Inspector Al Stewart told the radio that some people had been arrested for looting.

The quake hit at 4:35 a.m. (1635 GMT) shaking thousands of residents awake, New Zealand's National Radio reported. Some 12 aftershocks have rocked the region since, ranging from 5.3 to 3.9 in magnitude, GNS Science reported on its website.

Prime Minister John Key, Carter and Energy Minister Gerry Brownlee were to fly to Christchurch to inspect damage and review the situation, officials said.

Key said his sister, who lives in Christchurch, messaged him that "they had had an enormous earthquake and it had been terrifying ... that it went on for so long and was so violent they were getting knocked off their feet."

Civil defence agency spokesman David Millar said at least six bridges in the region had been badly damaged, while the historic Empire hotel in the port town of Lyttelton was "very unstable" and in danger of collapse. Several wharves at the port had been damaged.

People in the city's low-lying eastern suburbs had been advised to be ready to evacuate after power, gas, sewerage and water systems were cut by the quake, Inspector Coleman said.

Resident Colleen Simpson said panicked residents ran into the street in their pyjamas. Some buildings had collapsed, there was no power, and the mobile telephone network had failed.

"Oh my God. There is a row of shops completely demolished right in front of me," Simpson told the Stuff news Web site.

Kiwirail rail transport group spokesman Kevin Ramshaw said 13 mostly freight trains had been halted on South Island lines, with some damage already confirmed to rail lines north of Christchurch.

Christchurch International Airport was closed after the quake as a precaution, as experts checked runways and terminal buildings, a spokesman said.

New Zealand sits above an area of the Earth's crust where two tectonic plates collide. The country records more than 14,000 earthquakes a year -- but only about 150 are felt by residents. Fewer than 10 a year do any damage.

New Zealand's last major earthquake was a magnitude 7.8 in South Island's Fiordland region on July 16, 2009 -- a temblor that moved the southern tip of the country 12 inches (30 centimetres) closer to Australia, seismologist Ken Gledhill said at the time.