AFP Photo/Francisco Cobo

At least 30 people died in a pre-dawn prison riot in the northern Mexican city of Monterrey on Thursday, as smoke billowed from the building, Agence France-Presse reports, quoting local media.

Riot police and ambulances were deployed at the Topo Chico prison as broadcaster Televisa reported that 30 died while Milenio television spoke of 50 victims, with inmates and prison guards among them.

The newspapers Milenio and Reforma reported that the riot broke out in an apparent escape attempt.

Families of the inmates flocked to the prison following reports of fatalities.

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.