Six people have been killed in southern Mexico in clashes between protesting teachers and police.
Another 50 people, including many police officers, were injured.
The incident took place in the state of Oaxaca, where two high-profile union leaders were arrested last week, accused of corruption.
Members of the CNTE union, which has a history of radical activism, have been blocking roads in southern Mexico ever since the union leaders were arrested.
The authorities said the police were trying to clear the roads when unknown gunmen began firing at both sides in order to create chaos and conflict.
The policemen involved in the operation near the town of Nochixtlan were not carrying guns, Mexico's National Security Commission said in a statement.
The state-owned oil company, Pemex, warned on Friday that it might be forced to close a refinery in the area if the highway linking Oaxaca to the capital, Mexico City, remained blocked