ATHENS, Greece - Hundreds of rioters fought battles with riot police in central Athens yesterday, two weeks after the fatal police shooting of a teenager set off the worst civic unrest in Greece in decades.

Yesterday's violence followed a memorial gathering in the evening at the site where 15-year-old Alexandros Grigoropoulos died Dec. 6, in the central Athens neighborhood of Exarchia.

The rioters, using the nearby Polytechnic - as the National Technical University of Athens is known - as a base, launched attacks against riot police, throwing rocks and gasoline bombs and erecting roadblocks around the university. The battles follow an established pattern of rioters attacking and then retreating in the face of heavy use of tear gas by the police.

Security forces are prevented by law from entering the grounds of the Polytechnic. An exemption is allowed if the university administration gives the go-ahead, but so far no permission has been given.

Grigoropoulos' fatal shooting touched a nerve among Greek youth, who took to the streets to protest what they see as random police violence. The arraignment of the two police officers involved in the shooting has not appeased the young, who are demanding that police be disarmed.

The protests have been fanned by perceptions of rampant corruption among politicians and poor employment prospects as the economy takes a turn for the worse after 15 years of growth.

While most of the protests have been peaceful, a violent fringe of self-styled anarchists has clashed with police and destroyed property.

Yesterday's clashes, which included a firebomb attack against the offices of a credit-data company in Athens, an attack against a city-sponsored Christmas tree in central Athens, and the occupation of a movie theater in the northern Greek city of Thessaloniki, have dashed authorities' hopes that protest fatigue would set in during the Christmas season.

Earlier yesterday evening, a group of masked men broke into the building housing the offices of Tiresias S.A., a company that keeps records of delinquent debtors and cardholders, and firebombed the company's offices. The fire was extinguished, but the company's offices were destroyed, witnesses said.

Yesterday afternoon, about 150 youths attacked the Christmas tree at Syntagma Square in central Athens, tossing garbage and hanging trash bags from its branches before clashing with riot police. At least three news photographers were injured by police wielding batons. The tree survived the attack.

The original Christmas tree had been burned to the ground Dec. 8, during the worst night of rioting in central Athens.