Hundreds of Mexican soldiers have taken over the police headquarters in the eastern Tabasco state, as part of an effort to curb drug-related violence.
The troops seized weapons from police, after the force had come under suspicion of working with drugs gangs.
Three people were detained in the raid in Villahermosa, the state capital.
Tabasco had largely escaped the drug violence that has blighted Mexico, but in recent months the state has become caught up in the crime wave.
Last week, a severed head was found in front of the police headquarters. It had apparently been left there as a threat by criminals.
Now the police themselves have come under suspicion of being connected to the drug gangs, says the BBC's Duncan Kennedy in Mexico City.
A similar operation was mounted in the north-western city of Tijuana in January.
In that operation 3,000 police officers had their guns taken from them to see if the weapons had been involved in gang violence.
They were returned after several weeks.
President Felipe Calderon has made the fight against drugs cartels a priority, and sent about 30,000 federal forces to a number of areas since taking office last December.
Last year at least 2,000 people were killed in drug-related violence across Mexico.