Twenty members of Golden Dawn, a Greek political party, including its leader, have been formally charged with belonging to a criminal organisation.
Their arrests by an anti-terrorism unit early on Saturday followed the killing of an anti-fascist rapper on 18 September by a member of the neo-Nazi party.
The BBC reports it was the first time a whole group of MPs have been arrested since military rule ended in 1974.
A number of other warrants are believed to have been issued. The arrests were made by the anti-terrorism unit.
Golden Dawn issued a rallying call via a text message, saying: "We call upon everyone to support our moral and just struggle against the corrupt system!"
Its call for members to gather outside the police headquarters in Athens was answered by some 200 supporters who chanted slogans and waved Greek flags.
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.
