At least two men have been shot dead in a drive-by shooting outside Greece's Golden Dawn party office in Athens, police said.
Two attackers on a motorbike seriously injured one more man in Friday's incident in the Iraklio neighbourhood, according to information published on the party's website.
Golden Dawn said in a statement that the victims were 20 and 23 years old, but police had not yet verified their identities and details on the shooting were not immediately available.
"The murderers - whoever they are - will be dealt with unsparingly by our democracy. Let everyone know this," government spokesman Simos Kedikoglou told reporters outside the prime minister's mansion.
Scores of police in riot gear cordoned off the streets surrounding the offices in the northern Athens suburb of Neo Iraklio and TV footage showed Ourania Mihaloliakou, the daughter of the party's detained leader, had rushed to the scene
The attack came after Greek government unveiled plans to toughen laws against racism and prohibit "malevolent" denial of the Holocaust.
The draft legislation presented by the Justice Ministry on Friday provides for maximum three-year prison terms, and fines up to 20,000 euros ($27,000), for people found guilty of inciting racist crimes.
Similar penalties would apply to those convicted of denying the significance of genocides, crimes against humanity and war crimes - in a way that could incite violence, hatred, or either threaten or insult ethnic, racial or religious groups.
The draft legislation will be forwarded to Parliament for debate.
Greece has seen a rise in violence targeting Asian and African immigrants since its financial crisis erupted in 2009.
The attack came amid a crackdown by the government on Golden Dawn party after a self-proclaimed Golden Dawn sympathiser stabbed an anti-fascism rapper to death in September.
The party rose from obscurity on an anti-immigrant and anti-austerity agenda to enter parliament for the first time
Its banner features a swastika-like emblem and its leader has denied the Holocaust took place, but the group says it is not neo-Nazi.
It denied accusations of violence and involvement in the rapper's killing.
The party's leader, Nikolaos Mihaloliakos, and two senior lawmakers were ordered detained last month pending their trial on charges of belonging to a criminal group.