A gunman fired a spray of bullets at the headquarters of the governing centre-right New Democracy party near central Athens early today, with one round hitting an office occasionally used by the prime minister.
No-one was hurt in the attack, but police cordoned off the area around the building on the busy Syngrou Avenue.
No group has yet claimed responsibility for the 3am attack.
Government spokesman Simos Kedikoglou said that a bullet hit the window of an office used occasionally by Prime Minister Antonis Samaras.
No party officials were in the building at the time.
The PM's official office is at another building in central Athens, where he spends most of his time.
Police said the attacker was believed to have had at least one accomplice.
Nine Kalashnikov assault rifle casings were collected from the office and experts were examining a car found abandoned and burned-out near the scene.
The conservative New Democracy heads the three-party coalition which is pandering to the country's EU creditors with deeply unpopular spending cuts and tax rises.
Over the weekend, arsonists firebombed the home of the brother of Mr Kedikoglou, no one was injured.
There were a total of 17 incendiary attacks over the weekend including on offices of nominally socialist partner Pasok and on a Crete office of leftist opposition Syriza.
An anarchist group claimed responsibility for attacking the homes of five journalists working for national media on Friday, saying coverage of the financial crisis was pro-government.