Greek police have arrested the leader of the far-right Golden Dawn party, Nikolaos Michaloliakos, on charges of forming a criminal organisation.
Three more Golden Dawn MPs, a leader in an Athens suburb and nine other party members have also been arrested.
The arrests come amid anger over the murder on 18 September of anti-racist musician, Pavlos Fyssas.
A man held for the stabbing told police he was a Golden Dawn supporter, though the party strongly denies any link.
One of the MPs arrested on Saturday was party spokesman Ilias Kasidiaris.
A number of other warrants are believed to have been issued.
Golden Dawn has called on its supporters to rally outside the police headquarters in Athens and has vowed to fight back.
A text message read: “We call upon everyone to support our moral and just struggle against the corrupt system!”
The killing of Pavlos Fyssas, 34 – whose stage name was Killah P – has sparked protests in Athens and across Greece.
George Roupakias, 45, who said he was a supporter of Golden Dawn, was arrested. He was charged with voluntary manslaughter and illegal possession of a weapon.
The government launched a crackdown, including raids on Golden Dawn premises.
Two senior police officials resigned for “personal reasons” after the killing and another two were suspended. Seven other police officers were suspended.
Greek police also arrested one of their own colleagues who was reportedly working as a bodyguard for the party, during a raid on its offices in the western town of Agrinio on Tuesday.
The government also began an inquiry into the activities of Golden Dawn, which won nearly 7% of the vote in 2012 elections.
Prime Minister Antonis Samaras has vowed not to let the party “undermine” democracy, and government officials say it must now be treated as a “criminal organisation”.
Golden Dawn, Greece’s third most popular party, has accused the government of carrying out a witch hunt over the Fyssas killing.
Mr Michaloliakos, 56, said last week: “Everyone is talking about us without asking us. They are throwing dirt from morning to evening, and these great Democrats are not giving us an inch to respond.”
On Friday, Golden Dawn threatened to pull its 18 MPs out of the 300-strong Hellenic Parliament.
Mr Samaras’s coalition, which has 155 seats, would then face by-elections.
In recent months, Golden Dawn has been accused of perpetrating attacks on migrants and political opponents – including an attack on Communist Party members earlier this month which is said to have left nine people in hospital.
Golden Dawn officially denies being a neo-Nazi movement, though its badge resembles a swastika, some senior members have praised Adolf Hitler, and its members wear black T-shirts and combat trousers at anti-immigrant demonstrations.