Anti-riot police raided Mexico City's historic center to remove striking teachers on Friday, using water cannons and tear gas to clear the area while protestors responded with firebombs.

Hundreds of officers flooded the Zocalo square after some 200 protestors disobeyed a deadline to vacate the area to make room for the nation's independence day celebrations this weekend, AFP correspondents said.

The teachers, many armed with sticks and wearing masks, had set trash on fire and placed metal barriers in adjoining streets to block the police from entering the Zocalo, home to the National Palace, Aztec ruins and the city's cathedral.

Police cleared the Zocalo within half an hour, an AFP photographer said.

But clashes continued on side streets with television images showing police using water cannons, while thousands demonstrated in one of the city's main boulevards.

Thousands of teachers had been occupying the Zocalo for the past three weeks, but most had left the plaza before the deadline, leaving plastic tarps and trash behind. Police put out several small fires. Local businesses had shut their doors before the raid.

Manuel Mondragon y Kalb, head of the federal police, had warned earlier that the officers would clear the Zocalo after the deadline passed "otherwise this will go on forever and we will never resolve the issue."

He warned that aggressive protesters would be detained. The police were not carrying firearms or truncheons, he said.

The government of President Enrique Pena Nieto had insisted that the traditional independence commemoration would take place as scheduled in the Zocalo on Sunday and Monday.

Traditionally, the president delivers the "Cry of Independence" from the National Palace balcony on the evening of September 15, shouting "Viva Mexico!" and waving the Mexican flag before a military parade the next day.

Pena Nieto signed an education reform this week that requires teachers to undergo mandatory performance evaluations. The teachers say the new rules violate their labor rights.

The dissident National Education Workers Coordinator (CNTE) union led several protests in the city for the past three weeks, disrupting traffic in the already congested megalopolis and blocking road access to the international airport in two occasions.

[Image via Agence France-Presse]