Twelve Mexican police have died in a mountain highway ambush hours after the severed heads of 10 people were dumped in a small town in a key illegal drug growing region.

Gunmen opened fire on Sunday evening on a police convoy, killing 12 officers and wounding 11 more, said Arturo Martinez, spokesman for the Guerrero state government said on Monday.

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The ambush took place on a rural highway near the town of Teloloapan, located in southern Mexico between the beach resort of Acapulco and Mexico City.

Earlier Sunday, the severed heads of 10 people were lined  along a street outside a slaughterhouse in the center of Teloloapan.

The region has been long used by drug gangs to grow marijuana. Surrounding Guerrero state has seen a spike in violence since last year as several major gangs battle over trafficking routes.

The La Familia cartel and its offshoot, Los Caballeros Templarios (The Knights Templar), are among the gangs fighting for territory in the region. The heads had been left with a message threatening the La Familia gang, local media reported.

More than 50,000 people, including more than 2,500 police and soldiers, have died in drug-related violence since President Felipe Calderon launched an army-led crackdown on the cartels after taking office five years ago.