The clashes in Athens and Thessaloniki coincided with a police security operation to prepare for Turkish President Recep Tayyip Erdogan's two-visit to Greece, which starts Thursday. There were no immediate reports of injuries.
Authorities said at least 22 people were detained for questioning in the Greek capital, where a pair of rallies drew several thousand participants. Some youths proceeded to hurl stones, flares and Molotov cocktails at police officers and set a parked car ablaze, police said.
The unruly rowdy demonstrators also blocked streets with burning trash bins and material taken from construction sites.
Similar scenes unfolded in Thessaloniki, Greece's second-largest city, where protesters threw rocks at police from the top of apartment buildings. locks.
About 2,000 police were deployed in Athens for the events marking the 2008 death of 15-year-old Alexis Grigoropoulos. A police officer shot the boy while he was out with friends in Exarchia, a central Athens neighborhood popular with anarchists.
The policeman who fired the fatal shot said he didn't intend to shoot Grigoropoulos. He was convicted of deliberate manslaughter and is serving a life sentence.
The teenager's death sparked riots across Greece that lasted for weeks. Athens was the hit the worst, with many stores, buildings, and vehicles in the capital smashed and burned.
Kantouris reported from Thessaloniki, Greece.
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