A powerful explosion went off at the Ministry of Press and Information in Athens on Saturday, causing extensive damage but no injuries, reports said.
The explosion, the second in one week, caused panicked residents to run onto the street in the residential area that houses the ministry near central Athens.
Police said the mechanical device went off nearly 12 minutes after an unknown caller contacted two private television stations, warning of the explosion.
The bomb attack took place exactly one week after a leftist group claimed responsibility for another explosion outside the Greek Parliament, near the Monument of the Unknown Soldier.
The leftist group Fire Conspiracy Cells said in a statement posted on the Internet that it targeted the building to hit "the well-secured temple of democracy."
Since police killed a 15-year-old demonstrator in December 2008, the Greek capital has been beset by violent protests and multiple bombings against police stations, banks and state agencies.
The violence over the last year has left one policeman dead and seven wounded, some seriously.
The leftist group is known for small explosions using homemade gas cannister bombs targeting government buildings and politicians' homes and offices.