An American Express employee stands in the middle of rubble caused by a time bomb explosion in Piraeus, Greece, on Tuesday

ATHENS, Greece (Reuters) -- A Greek urban guerrilla group said on Tuesday it bombed branches of Barclays Bank and American Express to protest against NATO and the European Union.

An anonymous caller, representing the leftist Revolutionary Nucleii group, told Eleftherotypia newspaper the attacks "were against the criminal NATO formation, the EU and their banking oligarchies which force a capitalist dictatorship."

The bombs went off shortly after midnight in the port of Piraeus, south of Athens causing damage but no injuries.

Both bombs consisted of dynamite and a timing device.

Police said the Barclays bomb damaged seven cars parked nearby and blew out the windows of neighbouring businesses.

Foreign interests have been targeted by anarchists and urban guerrilla groups in Greece in the past in attacks ranging from arson on cars to relatively sophisticated time bombs.