Thousands of mourners gathered in an Athens cemetery for the funeral of musician Pavlos Fyssas who was stabbed to death by a member of the fascist Golden Dawn party.
Anti-fascist activist Mr Fyssas died early on Wednesday from two stab wounds to the chest after leaving a cafe in the western area of Keratsini.
Police arrested a suspect at the scene, who they say admitted to the killing and identified himself as a member of Golden Dawn.
The attack sparked outrage, drawing widespread condemnation from across the political spectrum and increasing calls for a crackdown on the fascists.
"This government is determined not to allow the heirs of the nazis to poison our social life, to commit crimes, to terrorise and to undermine the foundations of the country that gave birth to democracy," said Prime Minister Antonis Samaras in a televised address to the nation yesterday.
Deputy Prime Minister Evangelos Venizelos said Golden Dawn had "violence as its priority and must be dealt with as a criminal organisation."
And Hannes Swoboda, president of the Socialists and Democrats Group in the European Parliament, urged Greek authorities to examine banning the party altogether.
"Golden Dawn's openly xenophobic, neonazi hatred even goes as far as murdering political opponents.
"This is shocking and intolerable by any standards and more so in a European Union country," he said.
Anti-fascists confronted police throughout the night in several Greek cities with rock-throwing demonstrators facing riot police firing tear gas and stun grenades.
In Keratsini, 130 people were detained during the clashes, with 34 of them later arrested.
Scuffles broke out in the early hours outside Athens police headquarters, where those detained had been taken.
Similar scenes played out in Thessaloniki, where about 6,000 demonstrators marched.
There were also violent clashes in Patras, Xanthi, Larissa and Chania on Crete.
Authorities said 41 people had been detained in Keratsini and 36 in Thessaloniki.
The suspect, who was not named - in accordance with Greek law - appeared before a prosecutor on Wednesday evening along with his wife, who was arrested on suspicion of concealing evidence.