A prison riot in northern Mexico left scores dead and dozens more injured on Thursday, as authorities sought to account for inmates in custody.

The violence broke out before dawn at the Topo Chico prison, and gunshots were heard by relatives of inmates who gathered at the prison gates. There are also reports that fires erupted as prisoners set mattresses alight.

But the situation has been brought under control and no prisoners have escaped, according to a Twitter post by the Nuevo Leon state government.

Jaime Rodriguez, the governor of Nuevo Leon, said 52 people were killed and 12 injured, though it was not clear how many, if any, prison guards were among the casualties.

"We are experiencing a tragedy," Rodriguez said following the pre-dawn riot.

He said the riot was not sparked by an attempted prison break but by gang warfare, including gang members from the infamous Zetas drug cartel.

Television footage showed police vehicles patrolling the streets surrounding the prison, but rock-throwing relatives of prisoners also pelted guards with stones inside the prison gates, demanding to know the status of their loved ones.

"I want to know that my daughter is OK," said one woman outside the prison gates. "She is in the infirmary. There are children in there."

Pope Francis to visit Mexico

The violence comes just days before Pope Francis is to visit another prison in northern Mexico. The pontiff will arrive in Mexico on Friday for his first papal visit to the country. Next week he will visit a prison in the border city of Ciudad Juarez, which was once one of the most violent cities in the world.

In recent years riots have been an all too frequent occurrence in Mexico's prison system, which is beset by overcrowding and gang violence.

More than a dozen people were killed and 65 injured two years ago during a prison riot in the central Mexican state of San Luis Potosi.

In 2012, members of the notorious Zetas drug cartel plotted an elaborate escape plan with prison guards that left 44 inmates dead in a Nuevo Leon prison.

Mexico's prison system is plagued by violence and cases of inmate control, which are symptoms of corruption and a lack of resources, according to a 2013 report by the National Human Rights Commission.

Based on visits and interviews at 101 of Mexico's most populated prisons, the report found that 65 of the facilities were run by inmates, not authorities.

bik/jil (Reuters, AP, AFP)