Self-styled anarchists and left-wing fringe groups have staged several acts of violence in the past week, including throwing petrol bombs at a central Athens police station and setting police cars on fire while officers were inside.
Stolen weapons
The assailants said the petrol bomb attack was in support of jailed comrades, one of whom had allegedly been beaten by prison guards.
Police are investigating whether the 9mm rounds used in the Nea Ionia attack came from several weapons stolen from police guards in the past 18 months.
A police official, who declined to be named, said: "It appears to be part of the anarchist attacks we have witnessed over the past few days, but we rule out nothing."
In January a guerrilla group known as the Revolutionary Struggle fired a rocket-propelled grenade at the US embassy in Athens, causing minor damage.