MEXICO CITY (Reuters) - A violent clash between members of a dissident teachers' union and police in southern Mexico on Sunday has left three dead and 45 injured, as law enforcement attempts to dislodge the protesters from blocking a local highway.

Emergency services officials in Oaxaca state confirmed the deaths and number of wounded, although it was not clear what sparked the violence. Mexico's National Security Commission, or CNS, reported that 21 federal and state police were among the wounded.

The federal police have escorted 120 tanker trucks carrying chemical waste from the nearby Salina Cruz refinery, owned and operated by state-owned oil company Pemex, the CNS added in a statement.

On Friday, Pemex warned it could be forced to shut operations at the refinery in "a few days" if the highway blockade persisted.

The Sunday clash, involving scores of masked protesters lobbing rocks at police who responded with tear gas, escalated several days after supporters of a splinter teachers' union took over a highway near the town of Nochixtlan, about 50 miles (80 km) northwest of the state capital, Oaxaca City.

Last weekend, police arrested the leader of the local teachers' union for alleged corruption, redoubling government efforts to impose its authority on an education reform that has sparked months of protests across the country.

Ruben Nunez, head of one of the most combative factions of Mexico's CNTE union, was detained on suspicion of receiving money of illicit origin.

Nunez is secretary general of the CNTE's Section 22 in Oaxaca, a long-standing hotbed of resistance to government efforts to reform the education system.

Similar, if less violent protests, have frequently caused chaos in Mexico City, and the CNTE has led efforts to resist the education reform, in particular its mandate to carry out evaluations of teachers.

(Reporting by David Alire Garcia and Luis Rojas; Editing by Peter Cooney)