Nikolaos Mihaloliakos, leader of Greece’s far-right Golden Dawn party, has been arrested on charges of forming a criminal organization.
Another Golden Dawn MP has been held and more warrants issued, Greek police say.
The arrests and warrants come amid anger over the murder on September 18 of anti-racist musician, Pavlos Fyssas.
A man arrested for the stabbing told police he was a supporter of Golden Dawn, though the party strongly denies any link.
“The Secretary General and one lawmaker of the Golden Dawn Party were arrested a short while ago after arrest warrants were issued,” Greek police said in a text message.
The other MP arrested was said to be Ilias Kasidiaris.
The killing of Pavlos Fyssas – whose stage name was Killah P – has sparked protests in Athens and across Greece.
George Roupakias, who said he was a supporter of Golden Dawn, was arrested.
The government launched a crackdown, including raids on Golden Dawn premises and the suspension of police believed to be sympathizers.
Police 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.
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.