Officials said the fire was deliberately started during an early-morning brawl between inmates in one of the crowded prison's five towers. Television footage showed part of the San Miguel prison in flames, billowing with black smoke, before the blaze was put out by firefighters.

Hundreds of frantic relatives of the inmates flooded to the prison gates, screaming their family members' names and imploring police to tell them who had survived. Officials later began informing the families of those killed.

"It is a hugely painful tragedy," President Sebastian Pinera said. "We cannot guarantee the number of dead will not rise." A regional governor said the official estimate was 81, down slightly after clarifying a hospital report.

Justice Minister Felipe Bulnes said the prison housed 1,960 inmates, nearly twice the 1,100 inmate capacity.

The president said it was the third-worst fire Chile has ever suffered and the worst-ever accident in the country's jail system.