Updated

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.

Protesters in the northern city of Thessaloniki 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.

In the Greek capital of Athens, police fired tear gas at hooded youths who threw rocks and other objects at riot police and smashed bus stops. At least 10 people were detained.

Another march was scheduled for later in Athens and a gathering was being held Thursday night on the spot where police killed Alexandros Grigoropoulos in Athens on Dec. 6, 2008.

Grigoropoulos was 15 when he was shot dead during a night out with friends in the central Exarchia district, 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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Kantouris reported from Thessaloniki, Greece