The device was planted on a motorbike parked in front of the building in the city centre, according to unofficial reports from the police.
Witnesses reported seeing burning cars and a cloud of smoke coming from the building.
There were no immediate reports of injuries. A witness told reporters that police evacuated the building before the explosion.
An unknown man allegedly called a television channel and a newspaper to warn of the bombing around 45 minutes before the explosion.
The courthouse was badly damaged in the blast and windows within a radius of almost 200 metres were broken.
The blast was heard across a large part of the Greek capital.
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