A bomb went off near the Athens Appeals Court early Friday, injuring nobody but causing property damage.
A guard spotted two individuals leaving a bag in the darkness before dawn near the building, police spokesman Theodoros Chronopoulos told local SKAI TV.
The attackers also shot at a court guard but missed the target.
No one has claimed responsibility for the blast yet, but according to police sources the incident is being investigated as a terror attack by local anti-establishment groups.
Shortly before the bomb blast, those behind the attack also called two local media outlets to warn that a bomb would soon explode near the court.
In recent months protesters have stormed the court building in attempts to stop the auction of foreclosed properties. Earlier this week parliament passed measures to impose heavier penalties on protesters.
Greece has been suffering from home-grown crime gangs over the past four decades.
Despite the dismantling of gang groups, a new generation has taken over especially after the start of the debt crisis in 2009 and the bailout programs.