Fighting between inmates left at least 31 dead and 13 wounded on Wednesday in a jail holding alleged drug gang members in northeast Mexico, authorities said.
Police and soldiers surrounded the Santa Amalia jail, in Altamira on the Gulf of Mexico, and eventually regained control, a federal police spokesman said.
Prisoners used makeshift weapons in the clash, which broke out in the men’s section of the jail in the south of the border state of Tamaulipas.
Thirteen inmates suspected of involvement in the fight were detained, state authorities said.
The jail was designed to hold 2,000 inmates, but it has a population of more than 3,000.
It lies in an area where the rival Gulf and Zetas drug gangs have been locked in a bloody turf war. Many of the inmates are doing time on drug-related charges.
Prison riots and jailbreaks are common in Mexico, where about 230,000 prisoners are behind bars, with 25 percent in overpopulated prisons.
Tamaulipas, which houses about 7,800 inmates, has seen a wave of breakouts and clashes in recent years.
Twenty inmates were killed and 12 wounded in a prison fight in October in the town of Matamoros, across the border from the US state of Texas.
Another seven inmates died and 12 were hurt in a clash that same month in a jail on the outskirts of the northern city of Monterrey.
One of seven states bordering the US, Tamaulipas is a prime battleground for the powerful Zetas and Gulf gangs, whose rivalry often leads to brutal clashes on the streets, as well as inside jails.
The Zetas, who have spread a wave of fear across Mexico in recent years, were accused in the near simultaneous breakouts of 32 inmates from three jails in eastern Veracruz State in September. Fourteen were recaptured.
Rising lawlessness as the cartels battle it out for lucrative smuggling routes into the US, as well as running clashes with Mexican police and soldiers, have raised concerns on both sides of the border.