Survivors of a supermarket fire that killed at least 320 people said they found exit doors locked -- and, in one case, welded shut -- as they desperately tried to flee the inferno, police and witnesses said Monday.

Police were investigating reports from witnesses that guards locked the exits to keep shoppers from fleeing with goods they had not paid for.

"There are several witnesses who saw how they shut the doors to the supermarket, and we also confirmed that the emergency exit was welded," said Paraguay's police chief, Humberto Nunez. Firefighters had found the main doors locked, he added.

President Nicanor Duarte called for a quick investigation "so those responsible are punished with the full force of the law." He said a majority of the exits appeared to have been locked.

Three owners of the Ycua Bolanos supermarket and three security guards were taken into police custody. The main shareholder, Juan Pio Paiva, said no orders were given to lock the doors. "It does not make sense in a fire of this magnitude for security guards to close the doors and stay inside," he said at a news conference.

Survivors of Paraguay's worst disaster in decades recounted how the midday blaze swept through the packed supermarket Sunday in a working-class neighborhood on the outskirts of the capital. Officials said the fire was caused by a gas explosion near the food court.

"I reached the door, but it was locked shut and I could not leave through it," Blanca Alcaraz said in a television interview from her hospital bed. "Then everyone rushed on top of us, and we were thrown to the floor."

A large number of children, who had accompanied their parents on regular Sunday shopping trips, were among the dead. Entire families perished together.

"I grabbed my little brother by the hand but then I let go because there were so many people. Then fire fell on my leg and I fainted," said Maria Elena, 9, who wore an oxygen mask as she lay in a hospital bed. Her brother was in intensive care.

The Health Ministry reported 256 people hospitalized, 70 of them in intensive care. The magnitude of the disaster stretched the public services of one of South America's poorest countries. Hospitals were overwhelmed by patients suffering from burns and lung damage.

People attend a ceremony for victims of a fire Sunday at a supermarket outside Asuncion, Paraguay. Three store owners and three security guards were taken into custody following reports that exit doors had been locked.