Mexican authorities said rival gangs fought inside one prison wing before dawn Thursday in the CERESO federal facility which holds more than 2,000 in all.

Officials initially put the death toll at five, but later said mutilated bodies were found strewn throughout the wing where shots were heard early Thursday.

Mexico's Roman Catholic church, meanwhile, condemned the killing of 73-year-old Luis Lopez Villa, a priest whose body was found in La Paz township east of Mexico City on Wednesday night.

Lopez is the third priest killed this year in Mexico, and the 18th religious cleric killed across the nation since early 2013.

The Mexican Council of Bishops launched prayers for Lopez and appealed for a halt to the country's endemic violence.

Acapulco - homicide city

Acapulco's fatal prison riot began "because of the constant dispute between rival groups inside the prison," said state security spokesman Robert Alvarez Heredia.

The federal prison is located in the once glamorous Pacific Ocean beach city where gang-related homicides have surged in recent years.

Acapulco ranks as the second-most dangerous city in the world, with 113 murders per 100,000 residents annually.

The jail fatalities coincided with a Mexico tour by the US Home Security Secretary John Kelly who on Wednesday visited President Enrique Pena Nieto.

Their talks came hours after a gun battle between rival gangs at Las Varas in Chihuahua state that left 14 people dead. 

Reuters said it had been told by sources that Kelly on Thursday had gone to Guerrero state to observed Mexico's efforts to eradicate opium poppies.

Prisons overcrowded, understaffed

A report issued last month by Mexico's governmental rights agency said many of the country's prisons are unsafe, overcrowded and understaffed.

Last February, a brawl inside a jail in the northern industrial city of Monterrey, between inmates aligned with the Zetas and Gulf drug cartels, resulted in 49 deaths.

In June, seven people were killed, including three police, when officers stormed the Ciudad Victoria prison in the northern state of Tamaulipas to regain control.

ipj/bw (Reuters, AFP, AP, dpa)