Masked youths clashed with police in central Athens Friday in the first major test of the Greek government's vow to crack down on violent protesters after nationwide riots last month.

A group of demonstrators broke away from a rally by thousands of students protesting government education reforms and hurled rocks and flares at riot police who responded with tear gas and flash grenades. At least six protesters were detained, an Associated Press reporter witnessed.

The rally was the first demonstration since conservative Prime Minister Costas Karamanlis vowed tougher policing to counter the "catastrophic violence" of nationwide riots last month sparked by the police's fatal shooting of a teenager on Dec. 6.

The new deputy interior minister, Christos Markoyiannakis, whose duties include overseeing the police, has promised a "zero tolerance" approach to crime.

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The violence in December left hundreds of stores and bank branches across Greece damaged or destroyed and added pressure on Greece's increasingly unpopular conservative government.

The rally was one of three demonstrations being held in the capital Friday. About 2,000 protesters staged a peaceful rally earlier against Israel's attacks in Gaza.

Students and academic staff were guarding buildings at Athens Polytechnic in the center of the city to stop troublemakers from using the campus as a base.

Masked youths in December stockpiled firebombs on the campus before attacking riot police during near daily clashes. Police are forbidden from entering university buildings by law.

"Once protesters enter campuses, the situation becomes more difficult, because there is the risk of injuries," the rector of Athens Polytechnic, Konstantinos Moutzouris, told The Associated Press. "The main thing is to stop them getting in."

Last month's riots revived fears of a resurgence of violence by shadowy far-left militant groups. On Monday, a riot policeman was shot and seriously wounded as he stood guard outside the Culture Ministry in central Athens.

Authorities matched recovered bullet casings to a weapon used by the domestic militant group Revolutionary Struggle.

The 21-year-old policeman is still in hospital in a critical condition, and underwent more emergency surgery early Friday, doctors said.