Several people were reportedly killed in the riot and fire at a prison in the city of Monterrey in northern Mexico early Thursday.
There were conflicting reports about the number of casualties in the incident.
Citing Milenio Television, Reuters reported death of at least 60 people, while the Associated Press reported that as many as 52 prisoners were killed and 12 others were injured, quoting Nuevo Leon Governor Jaime Rodriguez.
The inmates' families, who gathered outside the prison, reportedly said they have been told that more than 50 people have died in the clashes and fire.
"We will stay here blocking this avenue until they give us an answer. We want to know how our relatives are doing because they are telling us that there are more than 50 dead and no authority is giving us answers," Ernestina Grimaldo, whose son is a prisoner, told AFP.
The incident occurred at around midnight when a fight broke out between the two rival groups in the prison, which is reported to be overcrowded. One of the groups involved in fighting was led by Zetas drug cartel member, the AP quoted Rodriguez as saying.
The fire started when some inmates set ablaze mattresses and a food-storage area, Prison director Gregoria Salazar told relatives, according to Reuters.
The fire has reportedly been put out.
The situation has been brought under control, Nuevo Leon state public security spokesman Antonio Arguello said. "The authorities formed a security cordon and nobody escaped. Everything was under control at 1:30 am," Arguello told AFP.
"We can't say how many people died until forensic services give us a report and the authorities finish counting inmates," he added.
In 2012, at least 44 prisoners died when members of Zetas drug cartel attempted to escape another prison in Nuevo Leon.