OAXACA, Mexico — Police fired tear gas Monday 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 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...