Teachers fought a pitched battle with state and federal police in the city of Oaxaca Tuesday, as the death toll from Mexico’s drug war continued to mount elsewhere in the country.

The confrontation in Oaxaca, the capital city of the southern state of the same name, was triggered by a visit by President Felipe Calderón, who came to sign some joint agreements with the state government and hold a meeting with local businessmen.

The confrontation broke out when demonstrating teachers attempted to enter Oaxaca’s Zocolo, or main square, and were turned back by helmeted riot police carrying weapons and shields. Tear gas and apparently live ammunition was fired against the protest.

The battle continued for some seven hours and left at least 20 people injured, several of them seriously, and a similar number arrested. Three journalists were among those hurt, one of them suffering a gunshot wound to the leg. Also seriously wounded was the head of the Free Union of Municipal Workers of Oaxaca, Marcelino Coache Verano.

The teachers were protesting against Calderón’s signing Monday of legislation that allows wealthier Mexicans sending their children to private school to deduct much of the tuition costs from their income taxes. The teachers and other opponents of the measure charge that it constitutes an escalation of the attacks on public education in Mexico and is in violation of the country’s constitution.

The demonstration was called by Section 22 of the National Education Workers Union (SNTE). The union struck 13,500 schools across the state on Wednesday, organizing a march through the center of the city of Oaxaca and blockades of highways in a number of areas. The union demanded the resignation of the state government’s secretary general, Irma Piñeyro, and the director general of the State Institute of Public Education, Bernardo Vázquez Colmenares.

The clashes revived the tensions that gripped Oaxaca for much of 2006, when an attempt by state police to break a strike by the same teachers union erupted into a general uprising that saw barricades erected throughout the city. At least 16 people were killed in the confrontations, which were quelled only after federal police occupied Oaxaca.

The new clashes underscore the extreme social and class tensions that underlie the increasing violence in much of the country as the Calderón government continues to prosecute a militarized US-backed “drug war” that has claimed the lives of over 34,000 people over the past four years.

A wave of killings in the northeastern border states of Tamaulipas and Nuevo León over the past several days were attributed to a struggle between the Gulf Cartel and the Los Zetas, a rival criminal organization, for control over drug routes into the United States.

In Tamaulipas, at least 18 people were reported killed and a number of others wounded on Monday. Eleven of the victims died in the town of Padilla. Gunmen also shot up the town’s municipal palace, police station and courthouse.

Meanwhile, in Nuevo León, the body of a top police intelligence officer was found in his burning armored vehicle. The officer, Homero Salcido Treviño, was the director of the Center for Integral Coordination, Control, Command, Communications and Computing (known as the “five c’s”), which coordinated drug war operations between the Mexican military and state and local police.

The deaths followed a weekend that saw seven people believed to be cartel gunmen, plus a bystander, shot to death in Monterrey, and another six killed in a grenade attack on a nightclub in Guadalajara. Also, on Sunday, seven people were gunned down in a working class neighborhood of Mexico City. Thirteen others were killed in the border state of Chihuahua, most of them in Ciudad Juárez, which has been occupied by the Mexican military.

On Tuesday, gunmen carried out an attack on an armored SUV carrying two special agents of the US Immigration and Customs Enforcement agency on the highway from Monterrey to Mexico City. One of the agents, Jaime Zapata, was killed and the other wounded. Homeland Security Secretary Janet Napolitano issued a statement saying the agents were “shot in the line of duty,” but did not indicate what they were doing in Mexico.

Napolitano and Attorney General Eric Holder announced the formation of an FBI-led joint task force to investigate the killings. The last time that a US agent was killed in Mexico was 1985, when Drug Enforcement Administration agent Enrique Camarena was murdered by a drug gang in Michoacan. Overriding obstacles to getting Mexican suspects extradited to the US for the crime, US agents resorted to abducting them and transporting them across the border, provoking protests from the Mexican government.

There is no doubt that this latest attacks will serve as a pretext for escalating the growing US intervention being carried out in Mexico under the mantle of the Merida Initiative, which allocated $1.5 billion in US aid to the country’s security forces for prosecuting a drug war.

Arturo Valenzuela, the US assistant secretary of state for the Western Hemisphere, testified this week before of House subcommittee, calling for “full fiscal year 2012 funding from Congress for the Merida Initiative with Mexico”, while describing the drug war as a matter of US “national security.”