Athens, Dec 7 - Violence escalated in Greece's two largest cities Sunday as protesters hurled petrol bombs and burning garbage bins at police to commemorate the first anniversary of the shooting death of a teenager by police.

Athens turned into a war zone as riot police fired tear gas at hooded youths hurling chunks of marble and petrol bombs after breaking away from a march of more than 3,000 people to mark the death of 15-year-old Alexandros Grigoropoulos by a policemen's bullet.

The rioters could be seen smashing bank windows and shops, setting garbage bins on fire and destroying cars. Officials said at least five protesters and sixteen policemen were injured in the clashes.

In the city's main square, police on motorcycles clashed with youths, who punched and kicked officers and threw them off their vehicles.

A small group of demonstrators broke into the Athens University's main building and tore down the Greek flag, replacing it with the red and black anarchist banner.

The dean of the university was rushed to hospital after he suffered serious head injuries during the break-in.

More than 300 masked demonstrators had been hiding out in the neoclassical building, ripping up the marble steps in front of the building to use as weaponry against the police.

More than 100 people were detained in both Athens and the Greece's second largest city of Thessaloniki.

On Sunday, the rioters smashed bank windows, overturned trash bins and set them alight as they hurled rocks and fire crackers at riot police. Authorities said at least 48 people were detained for public-order offences in Athens and another 50 in the northern city of Thessaloniki, where a similar demonstration also degenerated into violence.

Fearing a repeat of the rioting that caused millions of euros in damage last year, the Greek government deployed more than 10,000 officers throughout downtown Athens.

Police said they would implement a "zero tolerance" policy in dealing with the demonstrations and a memorial service in the Ezarchia district planned for Sunday evening and Monday. (dpa)