At least 13 people were killed in three gun battles in Matamoros, a smuggling springboard on the US-Mexico border coveted by drug gangs, state officials said.
In one shoot-out, Mexican Marines clashed with armed civilians traveling aboard several vehicles. Four of the civilians in the vehicles were killed, and the Marines confiscated three automatic weapons and ammunition, the state of Tamaulipas said in a statement.
Four more armed civilians traveling on vehicles were killed in a separate clash with Marines on a Matamoros street, and four men and a woman were killed in a gun battle between two groups of civilians, the statement read.
The dead people have not been identified, and the Tamaulipas statement did not say if they belonged to any criminal organization.
Rival drug cartels have battled for years to control the lucrative smuggling routes through the city, located in far north-eastern Mexico on the Rio Grande river across from Brownsville, Texas.
Tamaulipas is one of Mexico's most violent states due to a turf battle between the paramilitary Zetas gang and their former allies, the Gulf Cartel.
Mexican forces in July and August captured the head of the Zetas, Miguel Angel Trevino Morales, and the head of the Gulf Cartel, Mario Ramirez Trevino.
More than 77,000 people have been killed in drug violence in Mexico since former president Felipe Calderon launched a crackdown against the cartels in late 2006.