A top Greek judge's car was destroyed in a bomb blast, one of three explosions in the Greek capital over the past 24 hours, police said.
An Athens tax office caught fire in a separate attack and a third bombing targeted a government-affiliated migration organisation.
The car of the chairman of the Council of States, Greece's top administrative court, was gutted in the explosion of a gas canister bomb that was planted under his service car early Thursday.
Nobody was injured in that attack.
Judge Panagiotis Pikrammenos had only been appointed to the position on Wednesday.
The anti-terrorist service launched a probe into the tax office bombing in the central district of Ambelokipi on Friday, believed to be the work of Revolutionary Struggle, a far-left group that fired a rocket at the US embassy two years ago.
Half an hour before the blast, which caused a fire, anonymous warnings were phoned to two Greek daily newspapers, the police said.
Revolutionary Struggle, which features on the European Union's list of terrorist groups, more recently launched two strikes against US-based banking group Citibank and attacks on police that nearly killed a young officer.
In the third attack, a gas canister bomb exploded outside the offices of the government-funded Hellenic Migration Policy Institute (IMEPO) on Friday, causing minor damage and no injuries.
Frequent bomb attacks in Athens and the northern city of Thessaloniki are often reported to be the work of extreme leftist groups or the anarchist movement.
Extremist hits against police and business targets intensified after police fatally shot a teenager in December, unleashing a wave of youth protests and violence which emboldened radical groups according to analysts.
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