Athens, Greece (CNN) -- Demonstrators massed amid tight security in central Athens on Monday for a protest to mark the second anniversary of the killing of a Greek teen by police.
Police shut down the city center to traffic for 24 hours as the protest loomed.
Even before the official beginning of the march, police fired "limited" amounts of tear gas at hooded youths who set garbage cans on fire, blocked roads, and threw stones and plastic bottles at officers, police said.
One person was slightly injured after being beaten up by the crowd who thought he was an undercover officer. His injuries were minor and he refused to go to hospital.
Earlier, youths threw stones at police and smashed shop windows in the city center, police said.
Demonstrators plan to march to parliament to mark the anniversary of the police killing of Alexis Grigoropoulos, 15.
The officer who shot and killed him in December 2008, sparking nationwide riots, was found guilty in October of culpable homicide, the Justice Ministry in Greece said.
The officer, Epaminondas Korkoneas, 38, was implicated in the death of Grigoropoulos, who died after being struck by a police bullet.
The country erupted in riots and demonstrations after the shooting and continued for several days afterward. CNN
