Children and women are reported to be among dozens of people killed after a church school for 500 pupils collapsed in a surburb of Port-au-Prince, the capital of Haiti.
Students lay crushed under blocks of concrete while crying and screaming parents tried to find their children among the debris of the La Promese school on Friday.
"It's like an earthquake," said Brazilian major general Carlos dos Santos Cruz, the commander for UN troops in Haiti aiding the rescue work.
"We don't know how many people are under the debris exactly but there are a lot because they were attending school."
More than 30 people, mostly pupils at the school, had been found dead, rescue workers said, and scores more rushed to hospital in critical condition.
Rescue workers used tools and their bare hands to dig through to children buried in the debris of the school in the hills above the capital.
"My son who is 15-years old, he's dead. He's my only son," sobbed 40-year-old Josiane Dandin. "I don't know what I'm going to do."
Another woman screamed for her missing 12-year-old daughter. "I don't know if she is dead or alive," she said.
One boy whose legs were trapped by debris begged rescuers to cut his feet off, a firefighter said.
The aid group Doctors Without Borders said it had taken 85 injured adults and children from the school, half of them with life-threatening injuries.
The bodies of the dead were pulled from the wreckage of the three-storey building by teams of rescue workers, the AP news agency reported.
The death toll could go far higher, Matt Marek, a spokesman for the American Red Cross in Haiti, said.
About 500 students from primary through to high school attend the school in Petionville, but officials said they were unsure how many were inside at the time of the late-morning collapse.
The impoverished Caribbean nation, which lacks sophisticated rescue equipment, is continuing to struggle with the destruction wrought by four tropical storms and hurricanes that hit in quick succession this year, killing more than 800 people and destroying 60 per cent of the crop harvest.