Greek police fired teargas on youths marching in Athens to mark the ninth anniversary of the killing of teenager by a police officer.
Around 2,000 police had been deployed for the commemorations of the 2008 death of 15-year-old Alexandros Grigoropoulos.
At least twenty three people were detained overnight- although there were no immediate reports of injuries.
The teenagers murder sparked the worst rioting Greece had seen in decades, when thousands took to the streets, torching cars and smashing shop windows.
The riots, that were also fuelled by anger over unemployment and economic hardship in the prelude to Greece's debt crisis, lasted for weeks.
On Wednesday night after the demonstration, around 200 men with covered faces moved to the central neighbourhood of Exarchia where Grigoropoulos was killed and attacked the police with rocks and molotov cocktails.
The clashes coincided with a police security operation to prepare for a two day visit to Greece by Turkish President Erdogan.
Protesters honored the memory of Alexandros Grigoropoulos, condemned police violence and demanded equal rights for all citizens.