|Government buildings, police stations and banks have been previous targets of leftists violence in Greece [EPA]
An anonymous call to a Greek newspaper has warned of a second bomb attack at a tax office in the Greek capital hours after an explosion shattered windows of buildings and damaged eight cars in the centre of the city.
Police vacated the building and conducted a search.
Speaking to Al Jazeera from Athens, Helen Skopis, a journalist, said: "The caller has not given any indication of when the bomb will explode."
She said that a Balkans security expert told her that "he believes that there is no bomb at the tax office and in his opinion this was either a farce or a test for the police".
There was no immediate claim of responsibility for the attack.
"It was a huge blast. I was in the kitchen with my grandchild and the whole building shook. We saw smoke and flames," an elderly woman who lives about 50 metres from the courthouse told Flash radio.
Spyros Vougias, the deputy transport minister, said: "We condemn the action, there is no point in trying to change the world by risking the death of innocent people."
According to the Balkans expert, the group Revolutionary Struggle may be behind the attack because ANFO, the material used in the explosive device, has been previously used by the group.
The blast, which happened at 06:22 GMT, damaged the facade of a court building, located on Riancour Street in the area of Ambelokipi in central Athens.
According to police, the blast caused no injuries.
"An anonymous call to a TV station warned that an explosive device on a parked motorcycle outside the court house would go off in about 40 minutes," said a police official who did not want to be named.
According to Al Jazeera's sources, the caller also gave them the motorcycle's licence plate number.
Police rushed to the scene, found the motorcycle with the bomb and then evacuated the court building before the blast.
Authorities believe a makeshift explosive device was placed on a motorbike parked outside the Athens courthouse.
A police official said that the fact that the caller gave a 40 minute warning along with the motorcycle's plate number indicated the perpetrators were not aiming to kill.
Greece has a decades-old history of leftist violence. Some groups became more active after riots in December 2008, triggered by the police killing of a teenager.
They usually target government buildings, police stations and banks, mostly at night, and make warning calls.
In November, a Greek anti-capitalist group claimed responsibility for sending parcel bombs to a number of embassies in Athens and to foreign governments abroad.
"They have their own agenda, they have no ideological background, they are just out there to create chaos, they are against the system," Skopis said of the Revolutionary Struggle.