|Relatives of inmates at Apodaca prison gathered at a security fence following the riot inside the prison [AFP]
Violence at a prison on the outskirts of Mexico's northern manufacturing city Monterrey has left at least 44 dead, authorities said.
Victims were beaten, knifed and pummelled with rocks when a fight broke out between rival gangs at the prison in the early hours of Sunday, with prisoners starting a fire by torching mattresses, authorities said.
Al Jazeera's Adam Raney, reporting from Mexico City, said a local official confirmed at least 12 more inmates were wounded in the fighting.
"This is one of the most deadly prison incidents in recent months and there have been quite a lot," said Raney. "It is unclear yet what started the riot."
Pablo Gomez, a spokesman for the state of Nuevo Leon, said authorities were still investigating the cause of the violence.
The prison houses some 3,000 inmates, said Jorge Domene, a senior Nuevo Leon state official.
Inmates at the prison in Monterrey, about 225km from the border with the US state of Texas, include members of Mexico's Gulf Cartel as well as the feared Zetas cartel. Authorities could not confirm if the fight was between these two cartels.
The prison was secured by around 6am and an investigation began shortly afterwards, a local government spokesman told Mexican news services.
Powerful drug cartels are fighting for control of smuggling routes along the US-Mexico border and rivalries often spread into the prison system, where prisoners held on federal drug charges are mixed with common criminals.
A fight between rival gangs in another prison in the north of the country left 31 inmates dead in January and a further 13 wounded.
About 50,000 people have died in Mexico in the past five years since President Felipe Calderon launched an aggressive attack on drug gangs. Over-crowded and inefficient prisons are plagued by corruption and mass escapes.