ATHENS - Greek youths and far-left groups demonstrated in Athens and other cities on Friday in memory of a teenage boy killed in 2008 by a police officer, an act that sparked weeks of riots.

In the capital, police counted almost 4,500 protesters, who marched behind banners reading "Break the chains of submission with the fire of revolt" and "Leave the youth in peace" to remember 15-year-old Alexandros Grigoropoulos who was killed by an officer on December 6, 2008.

"These days belong to Alexis," the demonstrators chanted.

They then marched on Athens' Exarchia quarter, where the teen was shot dead, throwing stones and Molotov cocktails at the police, who retaliated with tear gas.

Three demonstrators were arrested. Traffic in a part of central Athens was blocked off for the night.

Authorities deployed 3,500 police in Athens, supported by drones, a helicopter and water cannon, a police source said.

The patrol officer who shot Grigoropoulos, Epaminondas Korkoneas, claimed he was acting in self-defense as he fired three shots in the bohemian Athens district of Exarchia, allegedly to keep back youngsters who were hurling objects at them.

Grigoropoulos was hit by a bullet in the chest and died before he could be taken to hospital.

For more than a month, hundreds of businesses were vandalized in Athens and other cities on the sidelines of protests by school pupils, university students, unions and left-wing parties.

In Greece's second city Thessalonika, about 2,200 people took part in Friday's march, police said.

A march in Patras, the country's third largest city, degenerated into violence with demonstrators throwing firebombs and the police firing teargas.

Grigoropoulos's mother Gina Tsalikian this week urged protesters to remain peaceful.

"Alexandros was a gentle, peaceable child, against violence," she told Open TV.

"Unrest and vandalism were foreign to his nature and insult (his) memory," she said.

In July this year, Grigoropoulos's killer Korkoneas was released from prison after his life sentence was reduced, having spent over a decade in jail.

The decision was controversial and last month, the Supreme Court called for his early release to be re-evaluated. -AFP