ATHENS — In a landmark verdict in Greece’s highest-profile political trial in decades, an Athens court on Wednesday found the neo-fascist party Golden Dawn guilty of running a criminal organization as it rose to prominence during the country’s financial crisis, systematically targeting migrants and left-wing critics.

The ruling came more than five years after the trial began in a makeshift courtroom in Greece’s largest high-security prison near Athens, and as at the beginning of the trial, none of the party officials were in the courtroom where the verdict was announced.

But thousands of Greek citizens had gathered outside the court on Wednesday, waving banners reading, “They are not innocent,” “Nazis out” and “Life terms to the murderers,” as some 2,000 police officers patrolled the area and helicopters and drones circled above.

The three-judge criminal court tied the party to a string of attacks including the fatal stabbing in 2013 of a left-wing rapper, Pavlos Fyssas. The party member who stabbed Mr. Fyssas, Giorgos Roupakias, was found guilty of murder on Wednesday.

Another five Golden Dawn supporters or members were found guilty of attempted murder over assaults on three Egyptian fishermen in 2012, while four were found guilty of causing bodily harm over assaults on members of Greece’s Communist Party trade union in 2013.

“Today is an important day for democracy,” the country’s president, Katerina Sakellaropoulou, 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.”

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Formed in the 1980s by a former Greek army commando, the Golden Dawn party was catapulted from obscurity to become Greece’s third largest political party at the peak of the country’s financial crisis in 2012, tapping into public discontent against austerity and a growing influx of immigrants.

During its time in Greek Parliament from 2012 to 2019, the party maintained public support by casting itself as patriotic and fighting the system from within, but it retained links with neo-fascist parties in other European countries and the United States.

A spate of violent attacks in 2013, culminating in the fatal stabbing of Mr. Fyssas, precipitated the party’s unraveling.