A Mexican federal police officer walks next to a burning car Thursday as he looks for protesters. The protesters are demanding the resignation of Oaxaca Gov. Ulises Ruiz, whom they accuse of rigging the 2004 election to win office and organizing bands of thugs to attack dissidents.

Oaxaca, Mexico ? Federal police surrounded a university packed with leftist protesters Thursday in conflict-ridden Oaxaca, clearing barricades and firing tear gas as the activists showered them with gasoline bombs.

At least 20 protesters, 10 officers and three news photographers were injured.

About 200 police in body armor and carrying riot shields advanced to the university gates and fought the protesters for more than six hours before they finally retreated.

Under Mexican law, the university rector must give the police permission to enter. Rector Francisco Martinez, speaking on the university radio station, called the operation an “attack” and demanded the police withdraw.

The federal police said they simply intend to “restore order and peace” on the streets and did not plan to storm the school.

Protesters with scarves covering their faces were seen running from the campus and lobbing gasoline bombs packed with nails, stones and firecrackers at police.

Officers hit back with water cannons and rounds of tear gas.

The university is a stronghold of the movement to oust Oaxaca Gov. Ulises Ruiz, who is accused of rigging the 2004 election to win office and organizing bands of thugs to attack dissidents.