A bomb exploded outside the offices of a Greek business federation in central Athens early on Tuesday, police officials said, smashing windows but causing no injuries.
Attacks against banks, politicians, journalists and businesspeople are not uncommon in Greece, which has a long history of political violence and has been mired in its worst economic crisis in decades.
Tuesday's blast, which police believe was carried out by domestic guerrilla groups, is the first such attack since leftist Prime Minister Alexis Tsipras came to power in January.
The device, triggered by a timer and placed in a backpack near the entrance of the Hellenic Business Federation offices, went off around 3:30 a.m. .
An anonymous caller warned a newspaper some 30 minutes earlier, a police official said on condition of anonymity.