12 others were injured during the 'pitched battle' that lasted 30 to 40 minutes at the Topo Chico prison in the northern industrial city of Monterrey, said Nuevo Leon state Governor Jaime Rodriguez.

The fight erupted following a dispute between leaders of two rival groups, including one led by a member of the Zetas drug cartel, Rodriguez said.

"They used sharp weapons, bats, sticks," the governor told radio Imagen, adding that the 60-year-old penitentiary houses 3,800 inmates, twice its capacity.

During the brawl, inmates set a fire in a supply room. TV images showed flames coming out of the prison in the middle of the night.

The riot 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.

Angry relatives flocked to the prison and tried to force their way in, desperate for information about loved ones caught in one of the deadliest Mexican prison riots in recent years.

Five of the injured inmates were in serious condition.

Rodriguez rejected speculation that women or children may have been inside at the time of the riot.

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