ATHENS (AP) — New protest marches were underway in Greece Thursday evening on the 10th anniversary of the fatal police shooting of a teenager, hours after violent initial demonstrations where masked youths attacked police with firebombs and stones.

Police said about 1,700 people were marching in Athens and the protest was so far peaceful. But other youths set fire to trash in the streets and set up barricades in the capital’s central Exarchia district — where Alexis Grigoropoulos was shot dead on Dec. 6, 2008, and which anarchists have adopted as a stronghold.

In the earlier protest in Athens, police fired tear gas at hooded youths who threw rocks and other objects at riot police and smashed bus stops. At least 28 people were detained. About 700 people took part in the march.

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 started during a midday march and continued through the afternoon, with vandals setting 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 in the center afterward.

Grigoropoulos was 15 when he was shot dead during a night out with friends in Exarchia, where many popular bars and restaurants are located. He was killed when a policeman fired his gun following an altercation. The policeman later said he had not aimed for the boy but that the teen had been killed by a ricochet.

The teenager’s death sparked the most extensive riots Greece had seen in decades. The two-week outburst of violence spread to cities across the country as youths smashed and looted stores, burned buildings and cars and set up flaming barricades in the streets.

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.

In Athens, authorities deployed about 2,500 police and a helicopter Thursday for the marches and rallies, while the main subway station at the central Syntagma Square was temporarily shut down.

Annual marches marking the anniversary of Grigoropoulos’ death often turn violent.

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By ELENA BECATOROS and COSTAS KANTOURIS , Associated Press

Kantouris reported from Thessaloniki, Greece. Nicholas Paphitis in Athens contributed.

Riot police try to avoid petrol bombs in the Athens neighborhood of Exarchia, a haven for extreme leftists and anarchists, Thursday, Dec. 6, 2018 after a rally commemorating the killing of a 15-year old student back in 2008. Demonstrators in Greece threw firebombs and pelted police with rocks Thursday as marches on the 10th anniversary of the fatal police shooting of a teenager degenerated into violence. (AP Photo/Yorgos Karahalis)