Riot police and ambulances were deployed at the Topo Chico prison while angry families of inmates seeking to enter the facility threw rocks over a gate.
The Televisa channel reported that 30 died while Milenio television spoke of 50 victims, with inmates and prison guards among them.
Milenio TV and Reforma newspaper reported that the riot broke out in an apparent escape attempt.
Authorities have yet to make any official statements about casualties.
Nuevo Leon state public security spokesman Antonio Arguello said the riot erupted at around midnight.
"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.
Some relatives of prisoners formed a line to block a boulevard.
"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," said Ernestina Grimaldo, whose son is a prisoner.
The incident erupted on the eve of Pope Francis' trip to Mexico, during which he is due to visit another notorious prison, in the northern border city of Ciudad Juarez.