An explosive device went off outside Greece's labour ministry in central Athens early on Saturday but caused no injuries, Greek police said.
A Greek newspaper received a phone call from an unknown caller that a bomb had been planted at the ministry and had informed the police, who cordoned off the area before the explosion, which caused damage to the building and broken windows, police said.
The caller said a previously unknown guerrilla group was behind the attack, a police official said on condition of anonymity.
Police have launched an investigation into the incident.
Greece has a history of political violence. Small bomb and arson attacks are frequent, and most of them do not cause serious damage.
However, the government said it was worried by the incident.
"The attack is very serious and has to do with a serious crime," government spokesman Pavlos Marinakis told Open TV broadcaster on Saturday.

Trump not satisfied with Iran's latest proposal for talks
Iran sends proposal for negotiations with US to mediator Pakistan
Iran threatens painful response if US renews attacks
Ukrainian drones hit Russian port of Tuapse for the fourth time
Detained Myanmar ex-leader Suu Kyi to meet legal team this weekend
