
The 5-year-long trial involved 68 defendants in four related cases.
Riot policemen try to avoid flames from a petrol bomb thrown by protesters after the Greek court ruled the Golden Dawn party as a criminal organisation, in Athens.
Golden Dawn supporter Giorgos Roupakias was previously accused in the stabbing death of anti-fascist rapper Pavlos Fyssas.
"Today marks a huge victory for justice and respect for Greece and the entire world", Eva Cosse, a researcher at Human Rights Watch, said. "Today's decision is a confirmation of the fact that democracy and its institutions are always capable of fending off any attempt to undermine them".
The party said it was the victim of a political witch-hunt.
The win emboldened party members who were already violently attacking migrants, saying Greece needed to be "cleaned" of foreigners. None of the 18 former lawmakers, who had all served the maximum 18 months in pre-trial detention at the start of the trial, were in court Wednesday.
Politicians from across the political spectrum, from the ruling Conservative New Democracy to the Greek Communist Party and the former ruling left-wing SYRIZA party, attended.
At its political peak around 2012, the party won 18 seats in the 300-seat parliament and three years later, sent three deputies to the European parliament. The extreme-right Golden Dawn party was set to make its debut in parliament after polling at between 6 and 8 per cent, well ahead of the 3-per-cent threshold needed to enter the legislature.
His killer, former truck driver Yiorgos Roupakias, had confessed to the crime, but the attack sparked outrage and the charges that Golden Dawn was a paramilitary-style organisation that used beatings, intimidation and murder as tactics - all with the knowledge of senior party members.
Anti-fascist protests were also held in other Greek cities.
Golden Dawn, for its part, has systematically denied any direct link to the attacks and has described the trial and charges brought against the party's leadership as "politically motivated" and a "conspiracy".
Amnesty International, which helped establish a system to record racially motivated violence in the country, said the ruling could enhance efforts to fight hate crimes.
"Pavlos did it. My son!" she screamed with emotion.
A far-right political party that was once the third largest political force in Greece has been declared a criminal organization in that country.
When the news of the guilty verdict was announced, the thousands outside the court erupted in cheers that echoed out from the avenue and the surrounding balconies overlooking the court.
This guilty verdict should be accompanied by the exemplary punishment of the guilty with the highest possible sentences and of course, they must go to jail.
More than 2,000 police officers were deployed around the courthouse Wednesday as some 20,000 protestors joined rallies organized by Leftist parties, migrants' associations and trade unions, according to police estimates, awaiting the ruling, amidst concern that in case of an acquittal there could be violent incidents. "Although the court's decision is an important step, there is still plenty of work to be done in Greece and beyond". "We are here because there is no room for fascism in our lives". The avenue outside the Athens courthouse is closed off to traffic and the building itself blocked off by a string of police buses.
The crowd at the anti-fascist rally waved banners with slogans such as "Fyssas lives, crush the Nazis".