A bomb exploded early Tuesday at a bank in southern Athens without causing any injuries, Greek police said.
The extent of the damage was not immediately clear. Police said the device went off at 4:05 a.m. (0105 GMT) at the entrance of a Eurobank branch on a main highway in the suburb of Aghios Dimitrios.
Half an hour earlier, an anonymous caller had phoned a Greek newspaper to warn of the impending blast. The man did not claim responsibility on behalf of a specific group.
But the target of the attack pointed to domestic far-left militants, who have become increasingly aggressive following the police shooting of a 15-year-old boy in December. The death sparked the country's worst riots in decades.
No one has been killed so far in the attacks, which targeted police, banks and a private TV station.