MONTERREY - Suspected Mexican drug hitmen dumped the head of a murdered man on top of a car in the street, police said on Friday, in a rare outrage in the wealthy city of Monterrey.

The head, found on Thursday night on the roof of a car parked in a middle-class residential area, had a written message next to it signed by the Gulf cartel, the country's most violent drug organization.

The ears were chopped off, a senior state police officer told reporters on condition of anonymity.

Mexican drug gangs, engaged in a bitter fight with each other and security forces, often behead opponents to scare rival traffickers but this was the first such decapitation in Monterrey, home to large corporations and a wealthy business elite.

The message, written on cardboard or paper, suggested the victim may have been a common criminal who had passed himself off as a member of the Gulf cartel's feared Zetas hit squad.

"This is what happens to people who want to pass for Zetas," the message read, according to El Norte newspaper. The man's body has not been found, police sources said.

More than 1,100 people have been killed in Mexico this year in drug violence.

President Felipe Calderon has sent 25,000 troops and federal police to fight cartels across Mexico since 2006 and pledged last week to take back Mexican streets from drug traffickers and gunmen.

Calderon this week deployed more than 2,700 troops to Sinaloa, the home state of kingpin Joaquin "Shorty" Guzman, Mexico's most wanted man .

Despite those efforts, six high-ranking police officers were killed in the past two weeks.