Greek police arrested the leader and more than a dozen senior members and lawmakers from the Golden Dawn party after the killing of an "anti-fascist rapper".
Party leader Nikolaos Mihaloliakos, spokesman Ilias Kasidiaris, three other lawmakers and more than 10 members of the party were arrested on charges of founding and participating in a criminal organization. "Nothing can scare us!," shouted a handcuffed Kasidiaris, flanked by hooded police officers carrying machine guns on his way to face a prosecutor.
In the most significant crackdown on a political party in Greece since the fall of a military dictatorship in 1974, the arrests are the biggest setback to Golden Dawn since it entered parliament on an anti-immigrant agenda last year.
Ranked as Greece's third most popular party, it is under investigation for the murder of Pavlos Fissas, who bled to death after being stabbed twice by a party sympathizer.
The detainees were taken under high security to the prosecutors' office. They are expected to be charged officially on evidence linking the party with a string of attacks, including the stabbing of the rapper on September 17 and the killing of an immigrant last year, court officials told Reuters.
The party has denied any links to the killing and Mihaloliakos has warned Golden Dawn could pull its 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 would become politically untenable, Mihaloliakos has argued.
Police also confiscated two guns and a hunting rifle from Mihaloliakos' home, saying he did not have a license for them.
Two police officials were also arrested on Saturday for breach of duty in relation to the case.
The party on its website called for protests.
Several hundred of its supporters gathered outside police headquarters waving Greek flags and chanting "Long live the leader!" and "Blood, Honour, Golden Dawn".
About 200 protesters unfurled a banner reading "Golden Dawn" outside the party's headquarters in Athens.
"Golden Dawn is here; It will not back down. You cannot jail ideas," Golden Dawn MP Artemis Mattheopoulos, who is not among those detained, told reporters.
TACTIC TO IMPRESS
Prime Minister Antonis Samaras's government has so far resisted calls to ban the party, fearing it could make it even more popular at a time of growing anger at repeated rounds of austerity measures and instead, it has tried to undermine the party by ordering probes that could deprive it of state funding.
Samaras ruled out snap elections after the arrests.
Finance Minister Yannis Stournaras said Greece did not face the risk of political instability and Justice Minister Haralambos Athanassiou said all Golden Dawn members who had been arrested would receive a "fair trial".
The arrests surprised Greeks wary of political theatre in a country where little has been done to rein in a party that is widely viewed as "neo-Nazi".
"It's good that they arrested them, but I'm afraid that we will start killing each other now," said Dimitra Vassilopoulou, a 58-year old housewife.
"Does the government actually mean it or is it just a tactic to impress us? Why didn't they do anything when the immigrants were killed? How come they just discovered that Golden Dawn is a criminal organization?"
Golden Dawn now controls 18 of parliament's 300 seats, scoring 14 percent in opinion polls before the stabbing. A survey by ALCO pollsters this week showed support had fallen to as low as 6.7 percent.
Greek lawmakers do not lose their political rights or seats unless there is a final court ruling against them but the government has proposed a law that could block state funding for Golden Dawn if police find links to Fissas's murder.
The party, whose emblem resembles a swastika, rose from obscurity to enter parliament last year after promising to mine Greece's borders to prevent illegal immigrants from entering. Its members have been seen giving National Socialist salutes but the party rejects the "neo-Nazi" label.
Human rights' groups have accused the party of being linked to attacks on immigrants but this is the first time it is being is investigated for evidence linking it to an attack.