MEXICO CITY - Six severed heads were left outside a junior high school in the northern state of Durango Monday in the latest shocking example of Mexico's drug trade-related violence.

That violence has seen authorities recover more than 150 bodies from mass graves in the past month.

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Officials said the heads were discovered in the early morning before classes started. They also confirmed that 11 bodies were found removed from mass graves that had been found in the state capital, Durango City.

The number of bodies removed from mass graves in Durango since early April has increased to 178.

Monday's gruesome discovery once again raised questions about public security in the country.

Poet Javier Sicilia, whose son was murdered in late March, led a four-day march to the capital over the weekend that attracted more than 50,000 demonstrators to the city centre for a rally Sunday. The demonstrators expressed their displeasure with the government's public security policies and the rising number of deaths attributed to organized crime and drug trafficking.

President Felipe Calderon on Monday expressed support for the demonstration.

"Mexico is going through complex moments in security matters," Calderon said. "I also want a Mexico in peace. I also want a Mexico without violence. I also want a Mexico free from the scourge and the oppression of criminals."

A government crackdown on drug cartels has claimed more than 35,000 lives since Calderon took office in December 2006.