Italy struck an early blow on Saturday against the country’s powerful ’Ndrangheta organised crime group, convicting 70 mobsters and others in a first, crucial test of the largest mafia trial in more than three decades.

Judge Claudio Paris read out verdicts and sentences against 91 defendants in the massive courtroom in the Calabrian city of Lamezia Terme.

While 355 people have yet to be judged in proceedings expected to last two years or longer, those judged on Saturday had opted for a speedy trial.

That procedure, which took place behind closed doors, allowed them to have a third of their sentence shaved off if they were convicted.

Since January, a specially adapted courtroom has hosted the “maxi trial” of hundreds of suspects affiliated with the ’Ndrangheta, the country’s richest and most powerful mafia group.

Famed anti-mafia prosecutor Nicola Gratteri – whose efforts to defeat the ’Ndrangheta have resulted in his having to live under police escort for more than 30 years – said the sentencing had gone “very well”.

Italian anti-mafia prosecutor Nicola Gratteri is pictured with his police protection. Photograph: Alberto Pizzoli/AFP/Getty Images

“Out of 91 defendants, there were 70 presumed innocent who were convicted,” Gratteri told Italian news agency Adnkronos, adding that those acquitted had been minor players.

Some of the group’s most dangerous members received the maximum 20-year sentence requested by prosecutors. They included Domenico Macri of the group’s military wing; Pasquale Gallone, the right-hand man of alleged mob boss Luigi Mancuso, whose trial is still pending; and Gregorio Niglia, whose role included procuring weapons and extortion.

About a third of the group received sentences of a decade or more, while 21 individuals were acquitted, seven at the request of prosecutors, Gratteri said.

The ’Ndrangheta is entrenched in Calabria, one of Italy’s poorest regions, in the toe of the peninsula’s boot. It has surpassed Sicily’s Cosa Nostra in power and wealth. The group controls the bulk of cocaine flowing into Europe.

The biggest fish in the prosecution’s case – Mancuso “The Uncle”, 67, considered the leader of the ’Ndrangheta families who dominated the Vibo Valentia province of Calabria; and ex-senator and lawyer Giancarlo Pittelli, 68, accused of being Mancuso’s white-collar fixer – have opted for the longer trial.

Of the eight defendants in the fast-track trial who faced a maximum of 20 years, six received the full sentence. They included Gallone, 62, who helped orchestrate his boss Mancuso’s three years on the run beginning in 2014. Mancuso had only recently been released from prison after serving 19 years.

The ’Ndrangheta has approximately 150 families jockeying for position within the organisation. They are supported by at least 6,000 members and affiliates in Calabria, swelling to thousands worldwide, experts estimate.

One of the cells inside a bunker room built for the maxi-trial. Photograph: Gianluca Chininea/AFP/Getty Images

Its reach is now international, with illegal gains reinvested in the legitimate economy. The ’Ndrangheta’s ability to infiltrate nearly all segments of public administration back home in Calabria has allowed it to reap lucrative contracts and cement its power.

Charges in the case include association with mafia, attempted murder, money laundering, usury, dealing drugs, extortion and illegal weapons possession.

The maxi-trial is being held in a sprawling courtroom to accommodate the hundreds of lawyers involved. More than 900 prosecution witnesses and 58 state witnesses have been called.

In comparison, Italy’s legendary maxi-trial of 1986-1987, which dealt a blow to Sicily’s Cosa Nostra, resulted in 338 people being convicted. Anti-mafia prosecutors Giovanni Falcone and Paolo Borsellino were later assassinated by the mob.