Harsh clashes in Athens during a protest of about 5 thousand people against police violence.
One officer was seriously injured in the head, security forces said.
The protest was sparked by a video, which went viral, showing an officer beating a man with a truncheon during a patrol to verify compliance with anti-Covid restrictions on Sunday.
Protesters threw stones and Molotov cocktails at a police station and officers responded with tear gas.
The incidents broke out in a suburb of the Greek capital on the sidelines of a demonstration of 5 thousand people against "police abuses", the police said.
A group of about 200 hooded men headed to the Nea Smyrni police station, where a young man was beaten with batons by an officer on Sunday during a check on anti-Covid containment measures.
They threw stones and Molotov cocktails at the police station and the police responded with tear gas and water cannons.
A dozen garbage cans were set on fire, forcing firefighters to intervene.
A policeman, seriously injured in the head, was taken to the hospital.
Journalists and photographers were also targeted.
Sunday afternoon in the square of Nea Smyrni, five kilometers from the center of Athens, the images of a young man on the ground beaten with a truncheon by a policeman, in the presence of at least three other officers, went viral.
In the video, the young man can be heard shouting: "I'm sick", while passers-by react indignantly at the police behavior.
According to police, the incident began when law enforcement was attacked by "thirty people who injured two of its officers", but many residents of Nea Smyrni denied it.
The judiciary has opened a preliminary investigation "to examine possible criminal acts by police officers".
The police also opened an internal investigation into the policeman being filmed.