The trial of leaders of Greek neo-Nazi party Golden Dawn was adjourned for a second time Thursday as they again refused to appear before judges in an Athens prison, calling into question how the historic case will be run.

In a repeat of their gesture of defiance when the trial opened in the high-security Korydallos prison last month, party leader Nikos Michaloliakos and his 12 MPs did not take their places in the dock.

The trial is considered the most important since the country's former military junta was tried in 1975

Sixty-nine party members and police officers are accused of murder, conspiracy to murder or being part of a "criminal organisation", a serious offence in Greece which carries a sentence of up to 20 years in prison.

The first hearing on April 20 was adjourned after two hours of legal argument because one of the defendants wanted to change lawyer. On Thursday, the presiding judge adjourned the case again until May 12 to consider a plea to move the case away from the specially-built courtroom inside the prison in a suburb of the Greek capital.

The judge declared it was up to the minister of justice to decide where the trial was held, while the minister has said it was the judges' decision.

Many have questioned the suitability of the prison for such a large and politically sensitive trial -- which is expected to take at least a year.

- Fear of clashes -

Local politicians also want it moved, fearful of clashes between Golden Dawn supporters and anti-fascist groups in the streets every time there is a hearing.

Nearby schools were closed on Thursday morning as anti-racist demonstrators held a protest outside the prison demanding the "conviction of the neo-Nazi murderers".

State prosecutors will try to prove that Golden Dawn operated as a criminal organisation with a military-style leadership that allegedly encouraged the beating -- and possibly the killing -- of migrants and political opponents.

Under the command of Michaloliakos, a 57-year-old disgraced former army cadet, the party has already been linked by investigating magistrates to at least two murders.

The 15-month inquiry into the party was sparked by the murder of a young anti-fascist rapper, Pavlos Fyssas, stabbed in September 2013 in Athens allegedly by a party member.

Fyssas's suspected killer is among the 69 standing trial, as are other party members linked to the attempted murder of four Egyptian fishermen in June 2012, and an attack on communist trade unionists in 2013.

Golden Dawn came third in Greece's general election in January, with 17 MPs in parliament.