Leaders of Greece’s far-right Golden Dawn party are among 69 people who have gone on trial on charges including murder and being in a criminal group.
The session began at a top security prison in Athens, without some of the key defendants present, and was adjourned until 7 May.
Witnesses were reportedly attacked by party supporters outside the jail, despite the high level of security.
Golden Dawn polled third in the January general election.
Critics describe the anti-immigrant party as a neo-Nazi group.
Dozens of anti-fascist protesters and supporters of Golden Dawn were kept apart by police outside the jail. But reporters said scuffles still took place, in which a witness and a protester were hurt.
All 18 of Golden Dawn’s MPs in the previous Greek parliament are among the defendants, but only a handful of them were in the specially-built courtroom for the start of the trial.
Leader Nikos Michaloliakos, 57, who was released recently from prison, is being kept under house arrest and was not in court on Monday.
The trial follows a lengthy investigation into the 2013 murder of anti-racist rapper Pavlos Fyssas by a Golden Dawn supporter. The singer’s mother was in court for the start of the trial.
Golden Dawn has also been linked to the murder of a Pakistani immigrant and beatings of political opponents.
The trial is due to last at least a year.