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 .
Alexis Grigoropoulos , 25 , was killed by a police bullet in December 2008 . Demonstrators clash with police officers even before the official march begins . A police officer was found guilty in 2009 of culpable homicide .