CHRISTCHURCH, New Zealand — One of New Zealand’s biggest cities lay in ruins today after a powerful earthquake toppled tall buildings and churches on a busy weekday, killing at least 65 people in the country’s worst natural disaster in decades.

The quake even shook a massive chunk of ice from New Zealand’s biggest glacier, some 120 miles to the east of Christchurch, where most of the damage was reported.

More than 100 people, including as many as a dozen visiting Japanese students, were thought to be trapped in the rubble as darkness and drizzling rain fellt.

It was the second major quake to hit Christchurch, a city of 350,000, in five months, though Tuesday’s 6.3-magnitude temblor caused far more destruction than a stronger September quake that struck before dawn on a weekend.

“It is just a scene of utter devastation,” Prime Minister John Key said after rushing to the city within hours of the quake. He said the death toll was 65, and may rise. “We may well be witnessing New Zealand’s darkest day.”