Eight people including a 13-year-old girl were killed in a string of shootouts between gunmen and security forces that caused panic in the border city of Matamoros, Mexican authorities said.
Seven of those killed on Sunday in the city across from Brownsville, Texas, were presumed criminals, according to a statement from the government of Tamaulipas state.
The girl was a bystander who was at a crowded shopping mall with her family during one of the shootings. The statement said she was struck by a bullet fired by one of the suspects.
The violence began in the afternoon with a traffic stop in which the occupants of an SUV with Texas license plates allegedly opened fire on authorities, and three suspects were killed. Authorities seized weapons, ammunition and packets of marijuana hidden in a suitcase.
That unleashed a series of chases and shootouts elsewhere in the city, as suspected criminals blocked roads with buses and other large vehicles to try to impede the movement of police and soldiers.
In the second incident, state agents shot dead three armed men who were travelling in two SUVs. More weapons were seized.
A group of suspects fled from there to the mall, where another firefight broke out with Mexican soldiers, and the girl and another gunman died.
Matamoros is a stronghold of the Gulf drug cartel, which operates in most of northern Tamaulipas.
In the southern state of Guerrero, another hotspot of violence, at least nine men were shot dead over the weekend after a private party in the municipality of Coyuca de Catalan, state prosecutors reported. As the party was winding down on Saturday, an argument broke out that escalated from words to weapons.