Oaxaca, Mexico ? Thousands of anti-government demonstrators marched through this tense colonial city Sunday, demanding the security forces abandon camps they set up last week to end a five-month protest.

Masked police officers clutching automatic weapons watched the protesters from rooftops as they marched to a plaza about a block from the encampments, yelling “Get out federal police!”

The leaders then formed a human chain to keep the crowd of 20,000 from confronting police, but about 400 people broke through and attacked the officers with stones and bottles. Some of the police lobbed rocks back, while officers on rooftops used slingshots to shoot marbles at those trying to confront the police.

A radio station at Oaxaca’s university, where the leftists had set up their base earlier this week, reported that gunmen had fired at some protesters near the university earlier Sunday, injuring a 21-year old student who was taken to a public hospital.

About 4,000 federal police swooped into the city on Oct. 29 to restore order following a five-month protest that had rattled President Vicente Fox’s administration, scared tourists out of Oaxaca and left at least nine people dead, mostly protesters shot by armed gangs.

After being chased out of the city center, the demonstrators moved to the university. Police surrounded the campus last week and battled hundreds of protesters armed with gasoline bombs, stones and fireworks stuffed with glass and nails, leaving more than 30 people injured.

Supporters cheer on a group of protesters during their march to downtown Oaxaca, Mexico. The protesters marched to the center of the city Sunday to demand an end to the federal police presence following their five-month protest to oust the state's governor.

On Saturday, masked protesters detained and blindfolded two men near the university, accusing them of being spies for the federal police.

Mexico’s Defense Ministry said in a statement Sunday that the men were soldiers who were tied up, beaten and robbed before being released. The ministry condemned the action but said it maintains its “commitment to the Mexican people” in “staying on the sidelines of the current situation occurring in the capital of the state of Oaxaca.”

The protests began in May when teachers struck for better pay and conditions in one of Mexico’s poorest states. When police violently broke up one of their demonstrations in June, protesters expanded their demands to include the ouster of Oaxaca Gov. Ulises Ruiz, whom they accuse of rigging the 2004 election that brought him to power.

Now the demonstrators also want the federal police to leave.

“They don’t guarantee security; to the contrary, they scare us and are rude,” said Jesus Velasco, 60, a businessman who was marching Sunday.