Masked youths smashed store windows and hurled rocks and firebombs at riot police who responded with tear gas Monday in a second day of violence during commemorations of last year's fatal police shooting of a teenager.

The death last year of 15-year-old Alexis Grigoropoulos led to some of the worst rioting Greece had ever seen, with gangs of youths smashing, looting and burning stores in cities across the country for two weeks, protesting heavy-handed police tactics.

Monday's clashes broke out during a demonstration by about 3,000 people, mostly high-school students, through the center of Athens. Several dozen youths at the tail end of the march attacked riot police with rocks, firebombs and firecrackers, smashing some of the bus stops, telephone booths and storefronts not damaged in Sunday's demonstration.

Protesters injured a passer-by who attempted to intervene, beating the middle-aged man unconscious. Police detained at least three youths.

Demonstrators also scrawled anti-police graffiti and stenciled a photograph of Grigoropoulos on store windows and walls along the demonstration route.

Minor clashes also broke out during a march of about 2,000 people in Greece's second-largest city of Thessaloniki to the north, where riot police fired tear gas to disperse youths pelting them with rocks.

Police said at least 16 police and five demonstrators were injured Sunday, while 177 people were detained in Athens and another 103 in Thessaloniki.

Police on motorcycles on Sunday chased rioters amid scenes of chaos at Athens' main Syntagma Square, with youths punching and kicking officers pushed off their bikes. One policeman who lost control of his motorbike struck and injured a female pedestrian, who was tended to by demonstrators until an ambulance arrived to transport her to a hospital.

At Athens University, masked protesters broke into the building, injuring the university's dean and pulling down a Greek flag flying on the neoclassical building, replacing it with a black-and-red anarchist banner. The dean remained hospitalized Monday.

The clashes continued late into Sunday night in the capital, while police also clashed with protesters in the southern city of Patras and the northwestern city of Ioannina.

On Sunday night, Civil Protection Minister Michalis Chrisochoidis defended tougher tactics used by police, despite criticism from a left-wing opposition party that said the government's response had been heavy-handed.

"Police detentions, when justified, are not illegal in a democratic society. Neither is it illegal for judicial officials to press charges," the minister said.

"Vandals and hooligans have nothing to do with democracy."