No one was in the New Democracy building at the time and no injuries were reported in the attack, the latest violent act in a recent spate of politically related trouble.
The shots were fired at approximately 2:30 a.m. local time and at least one bullet went through the window of an office sometimes used by Prime Minister Antonis Samaras, the BBC reported.
The attack came after assailants set off an explosive device Sunday at the home of the government spokesman's brother, Ekathimerini.com reported. No one was injured in that incident.
At least 17 bombs were set off during the weekend at several New Democracy, Panhellenic Socialist Movement and Coalition of the Radical Left buildings, authorities said.
A group called the Militant Minority, a suspected left-wing anarchist organization, claimed responsibility Friday for attacks on the homes of five journalists, ekathimerini.com reported.