The violence broke out in the Exarchia district of the Greek capital, where 15-year-old Alexis Grigoropoulos was shot dead on Dec. 6, 2008, and which anarchists have adopted as a stronghold.
Protesters set a car alight and hurled rocks, Molotov cocktails and fireworks at lines of Greek riot police, who responded with tear gas and a water cannon.
The clashes broke out after a peaceful march by about 1,700 people through central Athens. Another gathering was planned later Thursday night at the spot where Grigoropoulos died.
An earlier midday Athens march by about 700 people also degenerated into violence, with police firing tear gas at hooded youths who were throwing objects and smashing bus stops. At least 28 people were detained.
Authorities deployed about 2,500 police and a helicopter Thursday for the Athens marches and shut down the main subway station at the central Syntagma Square.
In the northern city of Thessaloniki, protesters set up barricades in the streets and hurled Molotov cocktails at police, who responded with stun grenades and tear gas. The running clashes went on for hours, and vandals also set fires at a subway construction site.
There was a lull in the violence during a peaceful evening protest in the northern city, but riots continued later.
Grigoropoulos had been on a night out with friends in Exarchia when he was killed as a policeman fired his gun following an altercation. The policeman later said he had not aimed at the teen but he was killed by a ricochet.
The ensuing two-week outburst of violence in 2008 spread to cities across Greece as youths smashed and looted stores, burned buildings and cars and set up flaming street barricades.
The policeman, Epaminondas Korkoneas, was convicted of murder and sentenced to life in prison. His appeals trial is ongoing. The second policeman present was sentenced to 10 years in prison and was granted conditional release several years ago.
Kantouris reported from Thessaloniki, Greece. Nicholas Paphitis in Athens contributed.
Copyright 2019 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.