A Greek extremist group has claimed responsibility for the Athens bomb attack last week and threatened visitors to the Olympic Games.
In last week's attack, cars were wrecked and windows shattered
The "Revolutionary Struggle" group, in a letter published in a Greek paper, said it was behind the 5 May attack.
Three bombs exploded outside a police station in Athens, injuring a policeman and causing damage to the building.
On Thursday, a small bomb exploded outside a bank in Athens, causing minor damage and no casualties.
Police say two similar devices were found at a nearby bank but did not explode. There has been no claim of responsibility for these latest blasts and no arrests.
The Revolutionary Struggle is known to Greek authorities.
In September last year, the group bombed a judicial complex in Athens, injuring a police officer.
In the letter published in the Greek weekly Pontiki on Thursday, the group criticised the security operation surrounding the event and threatened visitors to the Games.
"With regard to the Olympic Games, we say that Greece's transformation into a fortress, Nato's involvement, the presence and activities of foreign intelligence units show clearly that (the Olympics) are not a festival like Games organisers say, but it's a war," it said.
"All members representing the international capital, global mercenary killers and state officials, as well as well-off Western Olympic tourists planning to be here, are not welcome."
The statement and latest blast came as Greek security forces began a three-day Olympic safety exercise ahead of the Games between 13 and 29 August.
Security is a major concern at the Athens Games - the first summer Olympics since the 11 September 2001 attacks in the United States.
Athens has allocated tens of thousands of police officers and army personnel to protect athletes and visitors.
A city-wide network of surveillance cameras will be in operation and aerial patrols will take place.
Public order minister George Voulgarakis has said the bomb attacks last week damaged Greece's image overseas in the run-up to the event.