A sharp series of about 20 aftershocks rattled New Zealand's earthquake-hit city of Christchurch overnight, and earthquake experts warned Tuesday that another powerful tremor might hammer the region in coming days.

Residents watch as a crane demolishes earthquake-damaged stores in Christchurch, New Zealand, on Monday. (Rob Griffith/Associated Press)The weekend's powerful 7.1-magnitude earthquake smashed buildings and homes, wrecked roads and disrupted the central city, though nobody was killed and only two people were seriously injured.

The city centre remained cordoned off by troops Tuesday, with only building owners and workers allowed in to begin clearing up the mess — with much of the centre taking on the mantle of a ghost town.

More than 100 aftershocks, ranging from magnitude 3.2 to 5.4, have rocked the region since Saturday's major quake.

Overnight, about 20 shocks including two of magnitude-5.4 rattled the city, and quake experts said aftershocks likely will continue for several weeks — and the worst of them may be yet to come.

"It is still possible that we'll have a magnitude-6 in the next week, and people ought to be aware of that, particularly if they are around structures which are already damaged," said Ken Gledhill, a monitor at the geological agency GNS Science. "For a shallow earthquake like this, they will go on for weeks."

Prime Minister John Key called off a planned nine-day trip to Britain and France, citing what he called the quake zone's continuing "instability."

Key, who toured the city's damaged areas over the weekend, said 430 houses and another 70 buildings, many of them older structures, were already earmarked for demolition because of damage caused by the quake. Around 100,000 of the region's 160,000 homes sustained some damage, he said.

"I was awestruck by the power of the earthquake and the damage it has caused in the city," he told reporters. "It was miraculous that nobody was killed."