The teachers took over the private radio stations after unidentified gunmen opened fire on a state government radio station they seized on August 1. A male teacher was reportedly taken to hospital after the shooting, but the extent of his injuries was not immediately known.
About 50 protesters simultaneously took over each of the private stations in an apparent effort to broadcast their messages.
The teachers - who have been on strike since May 22 - refused to halt their work stoppage on Monday to allow 1.3 million public school students to return to classes at the start of the new school year.
One of the teachers in control of the radio stations broadcast a warning: "It would be better if you didn't bring your children to school."
The strikers, armed with crude weapons, blocked off all the main avenues in the centre of Oaxaca City and burned several vehicles overnight.
The 70,000 teachers originally went on strike to demand pay rises totalling about $125 million, but the government offered them less than one-tenth of that amount.
The protests have since expanded to demand the resignation of Governor Ulises Ruiz, whom the demonstrators accuse of rigging the state election in 2004 and of using force to repress dissent. Ruiz belongs to the Institutional Revolutionary Party which has governed the state since 1929.
Vehicles were set on fire by
strikers in Oaxaca City
The strikers have used the radio stations to denounce officials, intellectuals, the news media and others they say have refused to support their cause.
"We're fed up with neoliberalism," one said, using a term for free-market economics. "We are fed up with gringo ecotourism."
Oaxaca City paralysed
Oaxaca City attracts thousands of Mexican and foreign tourists each year because of its colonial architecture and local Indian crafts but the strike has paralysed the city centre causing tourism revenues to suffer.
The protests have erupted in violence on several occasions and one demonstrator was shot dead earlier this month. The demonstrators have taken control of radio stations and blocked news media offices in the past to protest against what they say is biased coverage of their movement.
News outlets also have been attacked by alleged government sympathisers. Earlier this month, gunmen opened fire inside the offices of a newspaper critical of the government, injuring at least two people.