A bomb exploded outside a tax office in Athens early on Friday, causing minor damage and no injuries, police said.
The blast occurred shortly after 3:00 am (0000 GMT) near a busy avenue in the north of the capital, just 10 minutes after a warning call was made to an Athens newspaper. Police had stopped traffic near the site.
Authorities confirmed that a bomb had been used in the pre-dawn attack but gave no other details of the device.
There was no immediate claim of responsibility. Far-left Greek militant groups have stepped up attacks in recent months, mostly targeting banks and government buildings. Greece's conservative government has promised to reorganize parts of the police force following the militant bombings and ongoing violence by local anarchist groups. It is receiving advice from London police.