Mexico's President Vicente Fox ordered federal troops into the southern city of Oaxaca on Saturday after a US cameraman for independent media website Indymedia and two Mexicans were shot dead.
Cameraman Brad Will was killed Friday in the state capital Oaxaca city by a bullet to the chest, said Florentino Lopez, spokesperson of the Popular Assembly of Oaxaca Villages (APPO).
Teacher Emilio Alonso was fatally shot three times in San Bartolo Coyotepec, 15 kilometres (10 miles) from Oaxaca City, while waiting for the governor of the state to arrive, his wife told local radio Universitaria.
Late on Friday, authorities also announced the death of Esteban Zurita Lopez, who was caught in the crossfire.
At least 11 people were injured by gunshots in the violence, including Mexican photographer Oswaldo Ramirez of Milenio magazine, who was shot in the leg, according to Lopez.
The shooting erupted around barricades erected by protesters, who have been fighting for pay raises for striking teachers and have demanded the resignation of the governor, Ulises Ruiz.
The shooting came in the fifth month of a strike by some 70,000 Oaxaca teachers for higher pay.
The government had hoped to settle the problem in negotiations before the inauguration of president-elect Felipe Calderon December 1.
Main access routes to the state capital, with a population of about 600,000, were blocked by barricades erected by the protesters.