MEXICO CITY, Nov. 2 (Xinhua) -- Mexican federal police on Thursday besieged a university packed with protestors in the troubled southern city of Oaxaca.

    The police used water cannons and repeated rounds of tear gas while protestors were running out of the campus and lobbing gasoline bombs at them. Some police officers also picked up rocks and bottles and hurled back at the protesters.

    At least 20 protesters, 10 police officers and three news photographers were injured. About six demonstrators were arrested,according to reports of the university radio.

    Rector of the university Francisco Marinez called the operationan "attack" and urged the police to withdraw.

    Since May, the protestors including trade unionists, leftists and Indian groups have been gathering in Oaxaca to press for theirdemand for the ouster of Oaxaca Governor Ulises Ruiz. They accused Ruiz of rigging the 2004 election and of using violence against his opponents.