A man died after unidentified gunmen opened fire on a radio station occupied by protesters in Oaxaca, southern Mexico, in an ongoing labour dispute.
Barricades blocked major roads into the city
Striking teachers seized 12 private broadcasting stations on Monday and set up burning barricades across the city.
Another man was injured in a similar attack on an occupied government radio station on Monday.
Teachers in Oaxaca have been striking for three months, demanding better pay and the state governor's resignation.
Hospital sources said one man died of gunshot wounds, and protesters said two others were abducted by the attackers.
The strikers were using the stations to tell parents to ignore Monday's start of the school year and keep children at home.
A spokesman for President Vicente Fox said that it was essentially a problem for the local state government.
"The presidency acknowledges... there is a serious problem in the case of Oaxaca, a problem that concerns the Oaxaca government," said the spokesman.
However, the Interior Ministry will hold talks with the teachers, their supporters and state government officials in Mexico City this week to try to end the conflict.
Governor Ulises Ruiz has become a key target for the teachers.
Although their dispute started out in May as a campaign for more pay, it has since transformed into an attempt to get the governor to resign from office.
The teachers say he is guilty of rigging the state election two years ago and of using heavy-handed tactics to deal with the strikers.
The governor, who belongs to the former ruling party, the PRI, has refused to step down.
Last week a number of people were taken hostage by activists after gunmen opened fired on a teachers' march.
They were later released.
The protests have also taken on a much wider context and have become woven into the continuing row over who won Mexico's presidential election, says the BBC's Duncan Kennedy.
Four months into this round of tension and parts of Oaxaca are starting to look ungovernable, and that could be a real challenge for the country's new leader.