Michoacan, Calderon's home state, has been one of the key battlefields in his crackdown on drug cartels.

The bodies were found piled up along a mountain highway late on Monday near the town of La Huacana.

Initial reports indicated the victims were likely killed over the weekend, when federal agents arrested Arnoldo Rueda Medina, a reputed chief of operations of the Michoacan-based La Familia cartel.

Police say his arrest on Saturday set off a string of brazen attacks against federal forces that left six federal police officers and two soldiers dead.

"This marks an important change in the drug war in that they are attacking federal forces directly," said Jorge Chabat, an expert on the Mexican drug trade.

"It also suggests the capture of this person has affected the operations of the cartel. It was a major blow and this is a reaction out of weakness not strength."

Calderon defiant

Calderon blasted what he called the "cowardly attacks" and declared that "the criminals will not be able to intimidate the federal government".

"In this battle we will not give up, we will not hesitate, because what is at stake is Mexico's peace and safety."

Federal forces arrested politicians in several Michoacan cities, including La Huacana, during an unprecedented sweep in May against officials believed to be co-operating with drug cartels.

Seven mayors, one former mayor and the state prosecutor remain in prison on charges of protecting the La Familia cartel.

An arrest warrant for alleged links to La Familia has also been issued for Julio Godoy, the half brother of Leonel Godoy, the Michoacan state governor, and a federal legislator for the leftist Democratic Revolution party.

Julio Godoy, who has gone into hiding, is allegedly one of several people in charge of seeking government protection for the cartel, national security spokesman Rubido told a news conference in Mexico City on Tuesday.