The violence, which appeared to be unrelated, escalated Wednesday in the western states of Nayarit and Michoacan, where drug cartels have been warring for territory.
Police in Nayarit initially responded to a citizen complaint of a kidnapping by a group of armed men, who fled on a federal highway near the town of Ruiz in the central part of the state, according the state prosecutors office.
As the officers headed toward the scene, they heard a second report of a shootout involving the same men, according to the statement, which did not identify the gangs or the victims.
Police found 28 men lying dead and four others wounded on the road littered with bullet casings from high-powered weapons and 10 abandoned vehicles.
The statement released late Wednesday by the attorney general's office gave no further details.
Earlier in the day, an official in the nearby western state of Michoacan said drug cartel violence had prompted frightened villagers there to flee hamlets and take refuge at five shelters set up at a church, event hall, recreation center and schools.
It is at least the second time a large number of rural residents have been displaced by drug violence in Mexico. In November, about 400 people in the northern border town of Ciudad Mier took refuge in the neighboring city of Ciudad Aleman following cartel gunbattles. That shelter has since been closed and most have returned to their homes.
Michoacan state Civil Defense Director Carlos Mandujano said about 700 people spent Tuesday night at a primitive water park in the town of Buenavista Tomatlan, with most sleeping under open thatched-roof structures.
Mandujano said state authorities were providing sleeping mats, blankets and food for those in the shelter.
Residents told local authorities that gunbattles between rival drug cartel factions had made it too dangerous for them to stay in outlying hamlets. The latest reports said arsonists were burning avocado farms in the nearby town of Acahuato.
"We woke up with fear (on Monday), but things appeared to have quieted down. It wasn't until later that morning that we saw SUVs with armed men driving by very fast and shooting at each other," said a woman who did not want to be named for security reasons.
Several displaced people said they would stay at the shelters all week before considering going back to their villages.
"I am not scared, but my children are," said a mother, who asked not to be quoted by name because of fear of retaliation.
The fighting in Michoacan is believed to involve rival factions of the Michoacan-based La Familia drug cartel, some of whose members now call themselves "The Knights Templar."
Mexico still has fewer people displaced by violence than countries like Colombia, according to the Norway-based Internal Displacement Monitoring Centre, which tracks such figures. It estimates about 230,000 people in Mexico have been driven from their homes, often to stay with relatives or in the United States. An estimated 3.6 million to 5.2 million people have been displaced by decades of drug- and guerrilla-war violence in Colombia.
Buenavista police chief Othoniel Montes Herrera said he has neither the manpower nor the armament to patrol rural areas frequented by drug gangs. Sending ill-armed officers out there "would be certain death, and we're not thinking of putting our personnel at that risk."
Drug violence has been on the rise in Nayarit, a Pacific Coast state known for its surfing and beach towns. In October, gunmen killed 15 people at a car wash in the capital of Tepic, an attack that police said bore the characteristics of organized crime. The bodies of 12 murder victims, eight of them partially burned, were found on a Nayarit dirt road a year ago. Officials have not identified the gangs fighting there.