Clashes have broken out between police and protesters near the site of the fatal stabbing of a left-wing activist blamed on an extremist right-wing party.
They came during a demonstration being held to protest at the killing of Pavlos Fyssas, a 34-year-old musician who friends described as an anti-fascist activist.
A 45-year-old man arrested for the attack has admitted to the stabbing and being a member of the far-right Golden Dawn party.
Earlier police raided the Golden Dawn offices. While the party has often been blamed for numerous violent attacks in the past, the stabbing is the most serious instance of violence directly attributed to a member of the extremist group so far.
Pavlos Fyssas, 34, died in a state hospital early after being attacked outside a cafe in the Keratsini area west of Athens.The 45-year-old man arrested at the scene admitted to attacking Mr Fyssas. A knife with traces of blood was found near his car.
The attack drew condemnation from across the political spectrum in Greece, and from abroad. "Golden Dawn's openly xenophobic, neo-Nazi 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," said Hannes Swoboda, an Austrian and president of the Socialists and Democrats Group in the European Parliament.
Golden Dawn, whose senior members have expressed admiration for Adolf Hitler although they deny being a neo-Nazi group, won nearly 7% of the vote in general elections last year. Recent opinion polls show its support has since risen to around 12%.
Party members and supporters, often clearly identifiable in black T-shirts and combat trousers, have been blamed for beatings and stabbings, usually of dark-skinned migrants, across the country.
In January, two men identified as party sympathisers were arrested for the fatal stabbing of a Pakistani migrant worker. The party's deputies have also frequently engaged in outbursts and name-calling in parliament, most recently insulting Muslim MPs as Turkish agents.
Golden Dawn, which denies it had any involvement in the stabbing, said party offices in the western city of Patras and on the island of Crete were attacked by people throwing petrol bombs later in the day.