Masked youths clashed with police in Athens on Wednesday, throwing Molotov cocktails and stones on the margins of an annual protest in remembrance of a teenage boy killed by police in 2008.
The small group of protesters also torched several cars, tore down street signs, toppled rubbish bins and set fire to debris in the Greek capital's central district of Exarchia.
School pupils, university students and left-wing organisations earlier held demonstrations in Greece's main cities in memory of 15-year-old Alexis Grigoropoulos, who was fatally shot by a police officer in 2008.
At the time, Grigoropoulos's death sparked days of youth unrest around the country.
In Thessaloniki, Greece's second largest city, protesters also threw Molotov cocktails as police used tear gas in clashes.
Greek police are already on alert over a two-day visit by Turkish President Recep Tayyip Erdogan that starts on Thursday.
The officer who shot Grigoropoulos following a street row, Epaminondas Korkoneas, was in 2010 found guilty of culpable homicide and sentenced to life in prison for the killing.
At his trial, Korkoneas claimed he had fired warning shots to keep back youngsters who were hurling objects at him and his patrol partner.
One of his bullets deflected and hit the boy's chest, killing him before he could be taken to hospital.
His patrol partner Vassilios Saraliotis was sentenced to 10 years for complicity but was conditionally freed in 2011.