There were similar incidents in the northern city of Thessaloniki and the rallies continued into the night.
Demonstrators clashed with riot police in Athens and the country's second largest city of Thessaloniki on Thursday during the first of a series of events to mark the 10th anniversary of the fatal police shooting of a teenager that sparked Greece's most extensive riots in decades.
In Athens, authorities deployed about 2,500 police and a helicopter for marches planned to remember Alexis Grigoropoulos, a 15-year-old who died on a night out with friends in the Exarchia district of the capital after being shot by a policeman following an altercation on December 6, 2008. Police responded with tear gas, and detained 10 people.
Protesters fought riot police in Athens on Thursday as there were gatherings to remember the death of Alexis Grigoropoulos ten years ago.
The 15-year-old died after he was shot by a policeman.
Al menos dos muertos al estrellarse una avioneta en Barcelona
El propio servicio de emergencias ha apuntado que los dos pasajeros del aparato han fallecido en el momento del impacto.
A group of hooded youths earlier broke away from the march and hurled stones at police who used tear gas to disperse them.
The policeman, Epaminondas Korkoneas, said he had not aimed for the teen, but that he was killed by a ricochet. 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. His appeals trial is ongoing.
Around 2,500 police officers and a helicopter were deployed for the Thursday protests in Athens, while the main metro station at Syntagma Square was temporarily closed.
Annual marches marking the anniversary of Grigoropoulos' death often turn violent.