CHRISTCHURCH, New Zealand — A powerful earthquake collapsed buildings at the height of a busy workday in New Zealand's city of Christchurch, killing at least 65 people and trapping dozens Tuesday in one of the country's worst natural disasters."It is a 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. "This may be New Zealand's darkest day," he told TV One News.It was the second time a major quake hit the city of 350,000 in five months. Tuesday's 6.3-magnitude temblor struck closer to downtown than a quake that heavily damaged Christchurch last September but caused no deaths when it struck before dawn on a weekend.Video footage Tuesday showed some multistory buildings collapsed in on themselves, and others with walls that had collapsed into the streets, strewn with bricks and shattered concrete. Sidewalks and roads were cracked and split, and thousands of dazed, screaming and crying residents wandered through the streets as sirens blared. Groups of people helped victims clutching bleedings wounds, and others were carried to private vehicles in makeshift stretchers fashioned from rugs or bits of debris.