Greek police have arrested the leader, two lawmakers and party members from the far-right Golden Dawn party, and charged one member with being an accomplice to the killing of an anti-fascist rapper.
They said the party leader Nikolaos Mihaloliakos and the others had been arrested on charges of founding a criminal organisation.
They are due to appear in court this weekend to be formally charged.
Golden Dawn, ranked Greece's third most popular party in opinion polls, is under investigation for the murder of Pavlos Fissas which sparked violent protests and a crackdown on political extremism.
The party has denied any links to the killing and Mr Mihaloliakos has warned the party could pull its 18 lawmakers from parliament if the crackdown does not stop.
If potential by-elections were won by the opposition, as some polls indicate, Greece's fragile two-party coalition could become politically untenable.