Some campaigners left the scene unconscious
Witnesses of the protests at Genoa say the police used excessive force in non-violent situations, but did nothing as a minority of extremists wrecked areas of the city.
The school, the campaigners say, was being used to train campaigners in non-violent direct action.
The protesters may have put up some resistance, but it is alleged that the police response was heavy and dramatic.
The occupants say they were made to stand against walls and were then beaten.
There have been allegations, in the Italian press, that the police sang fascist songs and urinated on captured protesters.
Around 20 people were taken to hospital - some were carried away from the scene unconscious. Another 20 walked out with severe bruising.
'Random beatings'
Campaigners tell of arrest for no good reason, inhuman conditions of detention, and in some cases further assaults after they were arrested.
"Random beatings, threats, cursing, separation, freezing cold conditions open to the air, many of us had loss of blood, and were made to lie on the floor with no bedding, " said British campaigner Dam McQuillan on his return to Britain.
Many of the protesters say they were asleep when the police entered the school, and that they were attacked without provocation.
Mark Covell is one of the protesters seriously injured in the events of Saturday night.
A full-time anti-globalisation protester, Mr Covell says he was co-ordinating peaceful protests when he was caught up in Saturday's police action.
"I though I was going to die. I could hear my bones breaking inside. My lung collapsed. Most of my ribs on my left hand side are smashed. My spleen is ruptured.
"That was just the first attack. Then the second one came in. They hit me again, just because I moved a bit. I just moved my arm, and they hit me again, sustained for about five or 10 minutes," Mr Covell told the BBC.
The BBC's Justin Webb, who visited the school after the attack said that it was clear that serious violence had occurred in the building, injuries received and blood spilt.
The question is whether the police were attacked first or whether this was a revenge attack by the police for the violent protests that had occurred earlier in the day.
The police have put on show the weapons they say they found in the building - petrol bombs, sledgehammers and metal bars. But those arrested say they had no violent intentions.