In Athens, authorities deployed about 2,500 police and a helicopter for marches planned to remember Alexis Grigoropoulos, a 15-year-old who died on a night out with friends in the Exarchia district of the capital after being shot by a policeman following an altercation on December 6, 2008.
Hundreds of demonstrators have hit the streets of Greece on the 10th anniversary of a teenager being fatally shot by an officer.
In Athens, protesters had set up burning barricades and pelted police with firebombs, rocks and fireworks, with police responding with tear gas, stun grenades and a water cannon. The running clashes started during a midday march and continued through the afternoon.
Police reportedly ran out of tear gas, with the riots the worst the country had experienced since the seventies.
On Thursday, a second march was scheduled for later in the day, while similar demonstrations were being held in Thessaloniki.
Protesters fought riot police in Athens on Thursday as there were gatherings to remember the death of Alexis Grigoropoulos ten years ago. The policeman later said he had not aimed for the boy but that the teen had been killed by a ricochet.
The teenager's killing sparked major riots which spread to cities across the country and lasted for weeks, with rioters smashing store windows, looting and setting cars alight.
The policeman, who claimed he had fired into the air when youths attacked him, was sentenced to life in prison. His appeals trial is ongoing.
Police deployed more than 2,000 officers in central Athens.
Annual marches marking the anniversary of Grigoropoulos' death often turn violent.