In recent days, small explosive devices were planted outside the homes of journalists and of the government spokesman's brother, while others targeted bank ATM machines.
Small armed groups have been active in Greece for decades, often planting bombs at night at company offices, but daytime attacks in public places are rare.
Last Monday, gunmen opened fire with an assault rifle on the headquarters of the conservative party, which heads the governing coalition. No injuries were caused, and there has been no claim of responsibility.
Demetris Nellas contributed to this report.