It was the second attack of its kind in two days targeting the Golden Dawn party, which rose from the margins of Greek politics to enter Parliament last year.
Police said the device detonated around 9 p.m. local time (1900GMT) Wednesday, damaging the door of the unoccupied seventh-floor office.
Nobody claimed responsibility for the attack.
Also Wednesday, an anarchist group calling itself "Angry Brigade" claimed responsibility for a similar attack a day ago in the city of Larissa. No injuries were reported in that attack.
Golden Dawn has repeatedly been accused of violence against immigrants, which it has denied.
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