Greek police have fired tear gas to disperse protesters hurling petrol bombs at them in central Athens.
The clashes broke out after demonstrations to mark the 10th anniversary of the killing of a teenage boy by police officers.
Hundreds had marched peacefully through the bohemian Exarchia district, where unarmed 15-year'old Alexandros Grigoropoulos was shot dead in 2008.
After the evening march, some of the protesters set garbage containers and cars alight.
People earlier laid flowers and lit candles during a memorial service on the spot where Grigoropoulos was killed.
Police had deployed more than 2,000 officers in Athens. Some in full riot gear formed protective cordons outside parliament and hotels in the city, while a helicopter hovered over the central Syntagma Square and neighbouring districts through the day.
At around noon, hundreds of students marched towards parliament.
On the night of December 6, 2008, Grigoropoulos had been out with friends in Exarchia when he was killed as a policeman fired his gun following an altercation.
The policeman later said he had not aimed at the teen but he was killed by a ricochet.
Hours after Grigoropoulos was shot, thousands took to the streets of Athens, torching cars and smashing window shops and looting.
The ensuing two-week outburst of violence spread to cities across Greece.
The policeman, Epaminondas Korkoneas, was convicted of murder and sentenced to life in prison.
His appeals trial is ongoing.
The second policeman present was sentenced to 10 years in prison and was granted conditional release several years ago.
The shooting, which triggered the country's worst riots in decades, was also fueled by anger over unemployment and economic hardship in a prelude to Greece's debt crisis.