Police say an officer was injured when masked attackers hit the Athens headquarters of Greece's left-wing governing party with Molotov cocktails.
A police official says 35 suspects have been detained for questioning in the Sunday's attack on the headquarters of the Syriza party.
The source requested anonymity because he was not authorised to speak about an ongoing investigation.
Syriza Central Committee secretary Panayiotis Rigas condemned the "murderous" attack.
Rigas says it was carried out by criminal thugs and wasn't ideologically motivated.
Five gasoline bombs were thrown and a car was burned. Opposition parties have stepped up criticism of what they claim is the government's lax policing after recent gas bomb attacks targeting buses and ticketing machines. They have accused Syriza of protecting the self-styled anarchist youth who commit the attacks.