Greece's anti-terrorism division has taken over the investigation into the murder of anti-fascist rapper Pavlos Fyssas, blamed on a supporter of the far-right Golden Dawn Party, the Public Order Ministry said.
The order followed the arrest of a 45-year-old suspect over the fatal stabbing early on Wednesday. In the wake of the killing, the government also appealed to a Supreme Court prosecutor to investigate more than 30 offences it says are linked to the ultranationalist party.
Golden Dawn, which has seen a sharp rise in support during the country's crippling economic crisis, has condemned the musician's killing and said it was not involved in the attack.
The party, whose members have referred to immigrants as "subhumans", won nearly 7% of the vote in general elections last year and has seen its popularity continue to rise in opinion polls to around 12%.
The killing of Mr Fyssas, a rapper known as Killah P, triggered protests around Greece and several smaller rallies in European cities, with the main march late on Wednesday turning violent.
An off-duty police officer has been held for questioning after holding up a gun while being chased by protesters during an anti-fascist rally in a central Athens suburb. Police said they were investigating reports that the officer had fired several warning shots.
Government officials said they were determined to prosecute violent far-right gangs, which they argued are being directed by Golden Dawn. "It is all beginning to unravel," government spokesman Simos Kedikoglou told private Star television. "We will dismantle this neo-fascist, this neo-Nazi monstrosity."
Police guards assigned to Golden Dawn's 18 members of parliament have been withdrawn, after the government described the party as being a "criminal organisation".
Party spokesman Ilias Kasidiaris said it has sued Public Order Minister Nikos Dendias for slander. "We are taking legal action against everyone ... who is responsible for this totally slanderous attack against a legal political party," he said.
"We have said from the very start that we condemn this horrendous attack and that it has absolutely nothing to do with our party. You are all liars and slanderers."