A bloody riot broke out on Sunday afternoon at a prison in Brazil's Amazon city of Manaus.
The violence ended 17 hours later when inmates surrendered their weapons and freed the last of12 prison guards taken hostage.
It's believed the riot was sparked by a fight between two rival drug gangs.
Head of security for Amazonas state Sergio Fontes says decapitated bodies were thrown over the prison wall by rampaging inmates.
"There were deaths, unfortunately. We have some outside the prison who were thrown from the prison by the inmates themselves. There've been escapes. We don't know yet how many. We're already looking for the escapees in the forest and highways."
Dozens of inmates have been killed in what is being described as the worst massacre in the state's history.
The death toll is expected to rise as authorities continue to assess the extent of the rebellion.
Family members of inmates waiting outside the Anisio Jobim Penitentiary Centre are desperate to know if their relatives are safe.
"My son is imprisoned in there. I want to know about my son, my son is there. Look at this bullet."
The mediator who negotiated the end of the riot with inmates, Judge Luis Carlos Valois, took to social media to describe the bloody scene inside the prison.
In a post on Facebook he wrote:
"I saw many bodies, it seemed that between 50 and 60 prisoners died, but it's difficult to say since many were quartered. When the police entered the compound, I returned home. I've never seen anything like it in my life, those bodies, the blood."
Officers believe a hole in the prison wall was used to smuggle weapons into the compound.
Head of state security Sergio Fontes says the massacre was planned well in advance.
"Everything indicates that the acts were planned over a long period of time. The inmates had established with the administration a promise that everything would run smoothly and there wouldn't be any problems during the holidays. They didn't keep their promise, but you can't count on criminals, can you?"
Brazil has the world's fourth-largest prison population in the world.
Many of the country's notoriously violent and overcrowded jails are run by criminal gangs fighting for control over several prisons.
Mr Fontes says violence between these rival gangs is not uncommon.
"What we've seen today, is what we've seen around the country. Gang fights. A larger gang attacking a smaller gang to completely eliminate the competition."
Another 87 inmates have escaped from a neighbouring prison in Amazonas.
One bold escapee posted a photo on Facebook as he fled the jail.
