Attackers shot at riot police guarding the offices of Greece's socialist Democratic Alliance party in central Athens early on Tuesday, slightly wounding one officer, Greek police said.
There was no claim of responsibility, though hardline left-wing activists have attacked the offices before.
The policeman was hit as he sat in a truck outside and taken to hospital, an official from the force said. Three rifle shells were found at the spot.
Small-scale attacks on businesses, police, politicians and embassies are frequent in Greece, which has a long history of political violence and is struggling to emerge from a huge debt crisis which has sent thousands to unemployment and cut incomes.
An organization calling itself the Revolutionary Self-defense Group said it attacked the party's offices in 2014, as part of a move against "the state oligarchy, the dictatorship of the capital and its armed guards".
Democratic Alliance condemned the shooting as a "murderous" attack on democracy.