Unidentified gunmen have fired shots at the Athens headquarters of Greece's ruling New Democracy party, in an attack condemned by all sides of Greek politics.
Police said a bullet pierced the window of the office of conservative prime minister Antonis Samaras, but no one was hurt.
Officials said Mr Samaras no longer uses the office and was not present at the time of the shooting.
Addressing supporters outside the building later in the day, Mr Samaras condemned the attack.
"You can shoot a person or at building, as they did, but you cannot shoot democracy," he said.
"Let them hear it then, those who must: Democracy will not be terrorised."
Government spokesman Simos Kedikoglou said even a symbolic attack on the prime minister was unheard of.
"This is a new, worrying escalation of the effort to create terror in our society," he said.
Greece is in the sixth year of a recession that has fuelled anger against the political class, and suspicion over the attack has fallen on left-wing anarchist groups.
It follows a spate of arson and vandalism attacks on other coalition offices and journalists' homes over the weekend.
Some experts are linking these attacks to recent police operations against squatters in public buildings.
The government says Syriza, the radical leftist main opposition party, tacitly backs anti-establishment groups and their attacks.
But party spokesman Panos Skourletis has denied that.
"This is certainly a dangerous escalation of terrorist attacks of blind violence, which are completely condemned by Syriza," he said of the most recent attack.