Thousands of striking teachers have forced up to half the country's public schools to close and vowed to stay out until their wages are doubled and their union is purged of corruption.

The National Education Workers' Coordinate, a dissident group within the National Education Workers Union, went on strike Monday claiming the support of about half of Mexico's 1.1 million public school teachers.There were no reports of violence.

"The strike will last indefinitely, until we obtain a positive response to our demands," said coordinate leader Teodoro Palomino. He said 8,000 schools in Mexico City and in 21 of Mexico's 31 states had suspended classes.

Education Secretary Manuel Bartlett threatened unspecified punitive measures against the teachers.

"We must all meet our responsibilities. If not, we suffer the consequences," he said Monday night in an interview on the Televisa television network. "It's not legitimate to strike to resolve internal labor problems. . . . It damages education in this country."

According to government figures, 25.4 million students attend public schools.

The coordinate on Sunday rejected a government offer to increase pay by 10 percent and fringe benefits by 18 percent.

Teachers make about $150 a month and President Carlos Salinas de Gortari has acknowledged that they are badly underpaid. But he says the government cannot afford the coordinate's demand of a 100 percent raise.

Coordinate leaders also demand "democratization" of the National Education Workers Union - Mexico's largest union - and the removal of union leaders they say are corrupt.

Jobs within the union have for decades been considered posts to reward loyal officials of Salinas' Institutional Revolutionary Party (PRI), which has governed the country since it was formed 60 years ago.