The Popular Fighters Squad (OLA), founded in 2013, had been the police's main suspect following Friday's blast.
The explosion occurred outside the Athens appeals court after calls warning of a bomb were made to media outlets, police said.
The blast broke several windows and caused extensive damage to the court's front entrance, where part of the first floor appeared to have collapsed.
Windows at nearby buildings were also shattered.
The attack occurred amid a bitter row over foreclosures on properties whose owners have longstanding debts to Greek banks.
The government has proposed new legislation which toughens rules against anti-foreclosure protesters, and the bill was approved by parliament last week.
Foreclosure sales are now increasingly being held online after successive protests, spearheaded by leftist groups, blocked property auctions over the past year.
The OLA carried out shooting attacks against the Israeli embassy in Athens in 2014 and the German ambassador's residence in 2013.
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