Protesters led by a teachers' union have blockaded a state-owned oil refinery in southern Mexico.

An official of Mexico's Pemex oil company said Monday the protesters had blocked roads leading into the plant, but had not entered the facility in southern Oaxaca state.

The official was not authorized to be quoted by name. He said the refinery in the township of Santa Maria El Tule had not shut down, but the protesters were preventing traffic in and out.

The protesters were demonstrating to mark the second anniversary of President Enrique Pena Nieto's administration, and to demand the government find 43 missing students.

The students were detained by local police in southern Guerrero state, who turned them over to a drug gang that reportedly killed the students and incinerated their remains.