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 by 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.
A pattern of political violence
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.