A strong explosion at the offices of the Panathinaikos supporters' club in the Athens district of Petralona early on Friday caused serious damage to the premises, adjacent stores and several parked cars.

The blast, which occurred at 3.30 a.m. and blew out the windows of several ground-floor stores and homes, caused no injuries but alarmed local residents.

Police cordoned off the area for several blocks and gathered evidence in a bid to trace the perpetrators behind the attack.