Petrol bombs and fireworks exploded in Athens on Thursday at demonstrations marking the 10th anniversary of the fatal shooting by police of a 15-year-old boy, which had triggered weeks of youth protests.
More than 4,000 people, mostly young people, joined protests in Athens as well as the northern city of Thessaloniki, according to police.
Demonstrations are held each year in memory of Alexis Grigoropoulos, who was killed by a police bullet on December 6, 2008, in a skirmish during street protests staged by Greek youths over conditions in the country on the verge of a economic crisis.
While the rallies in Athens throughout Thursday were generally peaceful, there were some disturbances as a group of hooded youths set up and ignited barricades and others threw stones at police who responded with tear gas.
More than 20 people were arrested, a police source said.
Greek police had already been on alert for the remembrance day, mobilising thousands of officers.
Epaminondas Korkoneas, the policeman who shot Grigoropoulos was in 2010 found guilty of culpable homicide and sentenced to life in prison -- a conviction he has appealed.
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 onto 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.