A explosive device went off overnight in front of the Greek labour ministry in central Athens without causing any casualties, police said on Saturday.
The explosion, which caused material damage, was preceded by an anonymous telephone call to the Greek daily Efimerida ton Syntakton about an impending blast, the police said.
The caller warned that an explosive device had been placed in front of the ministry which would be detonated 40 minutes later.
According to media reports, the man claimed to be a member of Revolutionary Class Self-Defence, a previously unknown organisation.
The area around the building was cordoned off by the police and the device exploded at 01:29 am (2329 GMT).
Public broadcaster ERT said the device had been placed in a bag left outside the gates of a bank near the ministry.
Attacks on public establishments, banks and diplomatic representations have been recurrent in Greece for years. They are generally attributed to extreme left-wing or anarchist movements.
The investigation has been handed over to the anti-terrorist services, police said.
"We are concerned about the attack, we have confidence in the police," government spokesman Pavlos Marinakis said.