Riot police and protesters clashed at Syntagma Square on Friday evening during a rally about Tuesday's fatal train accident in central Greece.
A group of protesters broke out of the main rally and threw flares, stones and other objects at the police, who responded with teargas, according to the Hellenic Police.
A short while later at Monastiraki, unknown individuals set fire to a police vehicle, injuring two police officers who were transferred to hospital.
Earlier on Friday, school and university students held a protest gathering at Propilea in central Athens, before marching to the headquarters of Hellenic Train on Sigrou Avenue, where their protest ended peacefully.
Several rallies as well as strikes have been held in Greece since the collision occurred late on Tuesday night near Tempi in central Greece between a passenger train travelling north and a freight train travelling south on the same line. At least 57 people have been confirmed dead, many of them college students. A station master in the central Greek city of Larissa has been detained over the accident.