More than 50 people have been killed in a riot in a prison in the Amazonian jungle city of Manaus sparked by a war between rival drugs gangs, according to Brazilian media reports.
Sergio Fontes, the head of security for Amazonas state, told a news conference he feared more dead would be found as authorities regained control of the Anisio Jobin prison.
After the riot broke out, six headless bodies were pushed over the permieter fence of the prison, the largest in the state.
Fontes said the riot was quelled by 7am local time on Monday (2 January), and authorities were still attempting to establish how many prisoners had escaped.
Brazil's prisons have been criticised by international watchdogs for overcrowding, and violent riots are frequent. The country has the fourth largest number of prisoners in the world, behind the US, Russia and China.
The clash took place between inmates aligned with the Sao Paulo-based First Capital Command (PCC) gang – Brazil's most powerful – and a local Manaus criminal group known as the North Family, according to Reuters.
The Manaus-based gang is believed to be carrying out attacks on PCC inmates at the behest of the Rio de Janeiro-based Red Command (CV) drug gang, the country's second largest.
Gang members from inner city prisons in Rio and Sao Paolo are often transferred to jails in provinces to break them up, however this has resulted in violence spreading, with 25 killed in a riot in a prison in Boa Vista, Roraima state, in October.
According to Brazil's Globo TV, the Anisio Jobin rioters held prison guards hostage, as well as 74 fellow inmates.
Some of the prisoners were executed and some released.
Footage obtained by Manaus' Em Tempo shows mutilated bodies piled on top of each other on the prison floor as inmates milled around.
The riot was one of the deadliest in years.
In 1992, 111 people were killed in Carandiru prison in Sao Paulo, most by police as they seized back control of the prison from rioters.
Thirteen people were killed in a riot in Anisio Jobin prison in 2002.