Al Jazeera's Nicole Itano, reporting from Athens, said the style of the attack was not an uncommon one in the city, usually meant to be "warnings" from far-left groups.

"Usually these bombs, which are placed outside of government building or banks - often there are warning calls before the bombs go off," she said. 

"The intent really, is to send a warning to the government, to the public, to large business, rather than to cause any sort of human casualties."