A court in Greece’s capital has ruled that the Golden Dawn party was operating as a criminal organisation, effectively banning a far-right group that was once the third-largest political force in the country.

Delivering landmark verdicts following one of the most important trials in Greece’s political history, the court on Wednesday said that seven of the party’s 18 former politicians, including leader Nikos Michaloliakos, were guilty of leading a criminal organisation, while the others were guilty of participating in it.

Those convicted of leading a criminal organisation face between five and 15 years in prison, while the others face sentences of up to 10 years.

The courtroom cheered as the high-profile trial reached its end after five years of hearings, as did thousands of people who gathered outside the appeals court in Athens holding banners reading “Fascism, Never Again” and “Freedom for the People, Death to Fascism”.

But the mood turned sour as police were quick to release tear gas and use water cannon to disperse crowds. Witnesses said people were seen gasping as the tear gas filled the air.

The marathon trial had been assessing four cases rolled into one: the 2013 fatal stabbing of Greek rap singer Pavlos Fyssas, physical attacks on Egyptian fishermen in 2012 and on left-wing activists in 2013, and whether Golden Dawn was operating as a criminal organisation.

The 68 defendants included the  former lawmakers from the party that was founded in the 1980s as a neo-Nazi organisation and rose to become Greece’s third-largest party in parliament during the country’s decade-long financial crisis.

Golden Dawn started to unravel in September 2013, when a party supporter was arrested for the killing of Fyssas. Its members, including Michaloliakos, were rounded up and authorities launched an inquiry into whether the group was a criminal organisation in parallel to the probe into Fyssas’s death. Golden Dawn failed to win a single seat in last year’s parliamentary election.

Before the court ruled that the party leadership was running a criminal group, the court found Golden Dawn supporter Giorgos Roupakias guilty of Fyssas’s murder.

Fyssas’s black-clad mother Magda, who has not missed a day since the trial started, burst into tears when the verdict went against Golden Dawn.

“Pavlos you did it,” she said. “We won a battle but are continuing the fight.”