CIUDAD JUAREZ, Mexico (AP) -- A mob of prisoners beat an inmate to death and violent clashes injured twelve others Thursday during a riot at a prison in this city on the U.S. border.

The prisoner uprising at the Social Re- Adaptation Center for Adults was the second in two days. On Wednesday, guards called in police, who used tear gas and fired their pistols in the air to restore order after a gang-related riot that lasted several hours but did not result in any major injuries.

On Thursday, members of rival gangs struggling for internal control of the prison rebelled again, triggering fights between a small group of prisoners that erupted into a large-scale brawl. The worst of the riot came while hundreds of friends and family members were inside the prison for visiting hours, said Oscar Valadez, a spokesman for state prosecutors.

The fighting lasted only about an hour, but was long enough for inmates to kill a 36- year-old man who was serving a 13-month sentence for robbery, said Arturo Herrera, the prison's warden.

Authorities at the filled-to-capacity, over 4,000-inmate prison, again called in police Thursday. Officers used tear gas and fired pistols in the air to restore order.

Police officers, federal agents and soldiers remained in the area around the prison and were on high-alert to guard against further disturbances late Thursday.

Battles between members of the "Los Mexicas" street gang and a rival faction, "Los Aztecas," have intensified in recent months at the prison, following an operation by authorities to crackdown on the trafficking of illicit drugs inside it, Valadez said.