Six people were arrested during raids by counter-terrorism officers in different locations in Attica on Tuesday morning.
According to reports, the suspects are associated with an extreme right-wing group calling itself Combat 18 Hellas.
The suspects are being investigated for participation in a criminal organization, for possession of explosives and arson attacks against migrants and leftist groups.
They are also suspected of vandalizing a memorial to Pavlos Fyssas, a musician who was stabbed to death by a member of the neo-Nazi party Golden Dawn in September 2013.
Police are also believed to be investigating whether the suspects are connected to the attack in February against an anti-fascist center in Piraeus that resulted in the serious injury of five people.
Combat 18 (C18) is a name which first emerged from the British ultra-right. Between 1998 and 2000 British police arrested many C18 members for a string of serious offences, including murder.
It takes its name from the first and eighth letters of the Latin alphabet – A and H – standing for Adolf Hitler.