Police in Greece say a small explosion was set off outside the offices of a left-wing human rights group in central Athens, damaging a wall but causing no injuries.

The blast occurred early Wednesday outside the offices of the Network for Political and Social Rights, which has campaigned for the rights of inmates and illegal immigrants.

Police say they believe a grenade was used in the attack. They said they received no warning or claim of responsibility for the attack.

Several of the groups members were inside the building when the explosion occurred. Politically motivated arson and small bomb attacks are common in Greece.