Demonstrators plan to march today from the university to police encampments in the center of the city as part of their five-month protest to oust the state's governor.

At least nine people have died since August in the unrest, which has rattled outgoing President Vicente Fox's administration. The planned march has sparked fears of more violence in the colonial city that was once one of the country's main tourist attractions.

Masked federal police clutching automatic rifles took rooftop positions above the city's main plaza, where other officers reinforced blockades. A commander briefed a group of police late Saturday.

"There is going to be a mega-march tomorrow. Be prepared. Keep your bulletproof vests on at all times," he said.

Protest leader Flavio Sosa, who is wanted by state police on conspiracy and riot charges, said the marchers will not look for a fight today, but he fears police may provoke one.

"Our enemies carry out murders, persecution and arbitrary arrests," Sosa told the Associated Press. "We have the right to defend ourselves."

Mexico's largest leftist group, the Democratic Revolution Party, has said it would join the protesters who want to form human chains around federal police detachments that enter the city.

The public university of 30,000 students in this southern city has been transformed into a stronghold for protesters since Fox sent in thousands of federal police last weekend to drive protesters from the city center, which they had seized. The demonstrators poured onto the campus after the police pushed them out of the main plaza, where they had camped out for months.