A judge handed down prison sentences ranging from five months to five years to the accused - who include police, prison officials and two doctors.
Another 30 defendants were cleared of charges, including assault.
All of those convicted are expected to appeal against the guilty verdicts.
The BBC's David Willey in Rome says it is unlikely that any of those sentenced will actually serve time in prison because their offences will have expired under Italy's statute of limitations before the appeal process is completed.
However, the Italian government will be forced to pay out millions of pounds to those who were victims of police brutality during their detention.
The 2001 meeting of the G8 in the northern Italian city of Genoa was one of the most violent in the group's history.
Street-battles between demonstrators and police left one protester dead and hundreds of others injured.
Police were accused of organised brutality at a high school where protesters were camping during the summit, and at a police barracks where demonstrators were taken after being arrested.
Among them were protesters from Italy, Britain, Poland and Ireland.
One of the prosecutors in the case, Patrizia Petruziello, said that 40 protesters who were arrested suffered "four out of five" of the European Court's criteria for "inhuman and degrading treatment".
The trial has lasted nearly three years.
Open hours at San Luca Offices: 3-5pm
Workshops and Chill Out Space at Burrida Social Centre: 6-10pm
The events will start on Fraday, June 18 and are all tbc