Mexico's President Vicente Fox ordered federal forces into the southern city of Oaxaca after further violence in a long-running dispute involving thousands of striking teachers.
Tear gas was fired at the crowds of activists who have been occupying the colonial city of 600,000 inhabitants.
Hundreds of officers then began forcing their way into the city, pushing the crowd back.
The trouble began in May, with a strike by teachers for higher pay. But they went on to demand the resignation of state governor Ulises Ruiz, accusing him of corruption.
The police were backed by armoured vehicles and water cannon as they pushed into the city.
Even burning barricades were not enough to halt the police advance.
The protesters - mainly striking teachers - were forced out of the city centre, ending a five month occupation.
The five month standoff had descended into frequent violence. In the latest outbreak, three people were killed including US journalist Brad Will.
With few police around, people sometimes took the law into their own hands and meted out their own punishments for alleged offences.
The strike has left more than a million children across the state deprived of their education.
Some parents have managed to make alternative arrangements - like this improvised classroom at a car wash centre.