Nine police officers were killed in an ambush in the western Mexican state of Jalisco and at least 10 more were injured, the state public security ministry said in a statement.
It was the fourth massacre this week, and brought the death toll to the week to 52. The dead included drug addicts at a rehabilitation centre, workers at a car wash and young people at a birthday party.
The officers were ambushed Thursday, as they drove in a five-vehicle convoy through a dirt road, the ministry said. The area is mountainous and only sparsely populated, but police had been called in to investigate the presence of armed men.
The officers were attacked with AK-47 and AR-15 rifles and with grenades. The attackers were travelling in at least 10 vehicles, the ministry said, and they escaped into the state of Michoacan, apparently taking with them a police officer who remains missing.
Over 9,000 people have been killed so far this year in Mexico in incidents linked to organised crime. Since December 2006, the death toll is over 28,300.
Over the past week, there have been three other massacres in Mexico.
On Friday, attackers who were apparently looking for a rival, killed 14 young people at a birthday party in a Ciudad Juarez home. On Sunday, 13 people were killed in Tijuana in a drug rehabilitation centre, in an alleged vendetta over the confiscation of 134 tonnes of marijuana by the authorities a few days earlier.
On Wednesday, 15 people were gunned down at a car wash in the western Mexican town of Tepic, in the state of Nayarit, which shares a border with Jalisco.