Thousands of striking teachers have seized control of the historic heart of Mexico City, blockading the central plaza armed with metal pipes and wooden clubs as riot police flood the area for what could be an ugly confrontation culminating weeks of protests against an education reform.

The teachers are using steel grates and plastic traffic dividers to block streets leading into the Zocalo plaza, home to the Metropolitan Cathedral, Templo Mayor and National Palace, some of the city's best-known tourist attractions. Hundreds of Mexico City riot police are massing on the other sides of the barriers.

Mexico's government has promised that Independence Day celebrations will take place there Sunday. The teachers are promising not to yield.