One of New Zealand's largest cities lay in ruins Tuesday night after a powerful earthquake killed at least 65 people, toppled buildings and left scores trapped beneath chunks of concrete.

Hundreds of rescuers were headed to the scene to aid in the search for survivors, according to CNN affiliate TVNZ.

"This is just heartbreaking," New Zealand Prime Minister John Key said during a trip to survey the damage. "This may be New Zealand's darkest day."

The 6.3-magnitude quake struck Christchurch during the lunchtime rush. Frantic rescuers scrambled to reach those trapped beneath the rubble hours after the earthquake hit. Dazed, bleeding residents wandered through streets strewn with debris and piles of concrete.