Authorities declared a formal civil defence state of emergency to coordinate recovery operations in the city, which has a population of about 350,000 people, after facades collapsed into streets, crushing cars and blocking roads.

Two men suffered serious injuries and police closed off the central business district.

The earthquake had a magnitude of 7.1 and a depth of 10 kms (6 miles), hitting the South Island city and a large surrounding area of farms at around 4.35 a.m. local time (1635 GMT Friday).

"There's a lot of damage that I've been able to observe in the central city area, mainly of the old brick and masonry buildings, a number of those have got walls that have fallen into the street," Christchurch mayor Bob Parker told Radio New Zealand.

The city's hospital said two men had been admitted with serious injuries, one hit by a falling chimney and the other cut by glass. It had also treated a few other people with broken bones, cuts and grazes.

Police said there were several instances of looting, which had been quickly contained. In the suburbs many houses had broken windows, toppled chimneys, cracked walls and items thrown off shelves.

Power was out over a large area of the city and surrounding region as circuit breakers were tripped at substations, but was being progressively restored after safety checks.

Water and sewage services were also disrupted, and there were reports of subsidence in some roads.

Rural epicenter
Officials were checking how severe the damage was in rural areas, closer to the epicentre, west of the city.

Ray O'Donnell, owner of a hotel in Darfield, a small farming community around 20 kms (12 miles) west of Christchurch, said large cracks had appeared in rural roads near the epicentre.

GNS Science, the New Zealand government seismological agency, revised its reading of the quake to magnitude 7.1 from an original 7.4. The US Geological Survey initially reported it at 7.4 but later revised its figure to 7.0.

The city's airport was shut as the runway and facilities were checked, and the railway network and bridges throughout the region were also being checked for damage.

The quake was felt as a long rolling motion lasting up to 40 seconds. The area was continuing to feel aftershocks as strong as magnitude 5.2.

"It was a real rocker, and (we're) still getting aftershocks. (It) felt like the house was flying on a whirlwind," Tessa Hay, who lives around 12 km north of the city, told Reuters.