Several off-duty federal police officers were shot dead early on Saturday in the city of Altamirano, in Mexico's western Guerrero state.
The exact number of officers killed had not yet been confirmed, an official with the federal police told Reuters.
Local media reported that between three and seven officers had been killed in the attack.
The media reports said seven officers were at a bar in the city shortly after midnight when heavily armed gunmen stormed in and opened fire without exchanging any words.
Also on Saturday, the bodies of seven unidentified men were discovered in the city of Uruapan, in Michoacan state, just north of Guerrero state.
The bodies were found sitting on plastic chairs, some with their hands and feet bound, just outside a Pepsi bottling plant, the state prosecutor's office said.
Written messages were attached to two of the bodies, warning that would-be criminals would face more violence, but they did not identify those responsible for the killings.
An estimated 70,000 people have been killed in drug-related violence in Mexico in the past seven years.
President Enrique Pena Nieto, who took office in December, has vowed to quell the lawlessness and killing that have stained Mexico's image as a tourist destination and rattled investors.