Police fired tear gas to prevent hundreds of leftist protesters from reaching the venue of an international folk festival in Oaxaca, in the worst outbreak of violence in the troubled Mexican city since November.

Protesters hurled rocks and burned vehicles on Monday as they sought to march to a stadium where the renowned Guelaguetza festival is scheduled to start July 23. Police responded with tear gas and rocks.

Some protesters said they only wanted access to the stadium to hold an "alternative," non-commercialized version of the festival, while others vowed to block the event entirely.

The picturesque colonial city was paralyzed by political upheaval for five months in 2006, when demonstrators essentially seized control of the downtown and prevented the Guelaguetza festival from being held.

The state government has vowed to defend the stadium and put on this year's Guelaguetza, an annual weeklong celebration of Indian music, artisan crafts and cuisine that dates back to the 1700s and draws tens of thousands of tourists from around the world.

"About 200 people wearing masks and carrying sticks, stones and bottle rockets began to provoke the police," the Oaxaca state government said in a statement. "The police repelled the attack using tear gas."

The Mexican League for the Defense of Human Rights, which has sided with Oaxaca protesters in the past, accused police of "brutally beating" the demonstrators and roughing up several reporters.

The league said about seven people were detained, and eyewitnesses said several were hit by flying rocks and tear gas canisters.

State public safety secretary Sergio Segreste said 30 people were arrested and 15 policemen injured, but offered no information on injuries to protesters.

The unrest began as a teachers' strike in May 2006, but quickly evolved into a broader protest as a coalition of leftist groups demanded the ouster of Oaxaca Gov. Ulises Ruiz. Ruiz denies allegations of rigging his 2004 election and sending armed thugs to attack his enemies.

Twelve people were killed, mostly protesters shot by gunmen, before federal police retook the city center in October.

In May, the National Human Rights Commission blamed both authorities and protesters for "excesses" during the conflict.