Eight people including a 13-year-old girl were killed in a string of shoot-outs between gunmen and security forces in the Mexican border city of Matamoros.
A Tamaulipas state government statement said seven of those killed in the city across from Brownsville, Texas, were presumed criminals.
The girl was a bystander at a shopping centre 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 with a traffic stop in which the occupants of an SUV with Texas licence plates allegedly opened fire on authorities.
Three suspects were killed.
That unleashed a series of chases, roadblocks and shootouts in which the other victims 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.