The Conspiracy of Fire Cells, which also claimed a wave of parcel bombs sent from Greece to embassies and foreign governments abroad in November, said it staged the attack to protest the recent arrest of some its members.
"We dedicate the attack at the court house to our brothers," it said in a pamphlet uploaded on a leftist internet website.
The group said it targeted the court ahead of the trial of 13 its members on January 17 and that it would continue to strike.
Police believed the claim was authentic but were still investigating, officials told Reuters.
The Athens courthouse was seriously damaged in the December 30 blast, which shattered windows of nearby buildings and wrecked at least eight cars.
There were no injuries as police had evacuated the area after an anonymous warning call that an explosive device on a parked motorcycle outside the court house would go off.
Greece has a decades-old history of leftist violence but some groups became more active after riots in December 2008, triggered by the police killing of a teenager.
Greek police have arrested more than 20 suspected members of guerrilla groups in the last two years.
Such groups target mostly government buildings, police stations and banks, mostly at night, and make warning calls.
The Conspiracy of Fire Cells initially specialized in arson attacks but turned to bombings in May 2009.
(Reporting by Renee Maltezou and Angeliki Koutantou; Editing by Jason Neely)
Reuters
http://news.yahoo.com/s/nm/20110105/wl_nm/us_greece_bomb_claim_1
