Police in the Mexican state of Oaxaca have fired tear gas at protesters in the state capital in the worst outbreak of violence there since last November.
The authorities insist this year's festival will go ahead as planned
Demonstrators set fire to buses and threw rocks as they tried to march to a stadium due to stage a major cultural festival from next week.
Dozens of people were reported to have been injured in the fighting.
Last year the Guelaguetza festival was disrupted by protesters demanding the resignation of the state governor.
They accuse Governor Ulises Ruiz of abuse of power and electoral fraud.
The Guelaguetza festival, which celebrates indigenous music, food and crafts, is held every July and is a major tourist draw in one of Mexico's poorest states.
Activists from an umbrella group known as the Popular Assembly of the Peoples of Oaxaca (Appo) said the festival should be for Oaxacans and not commercialised.
"The struggle and the boycott of Guelaguetza continues," Appo spokesman Florentino Lopez told a local radio station.
Police said Monday's violence was started by some 200 people "wearing masks and carrying sticks and stones".
The centre of the state capital was blocked for several months last year before being retaken by police in October.
Last year's festival was cancelled amid the violence.