“Relations with manager [Dennis] Katz were crumbling fast, a process that climaxed in Milan on February 13, 1975. Reed was in Italy to launch the first leg of a world tour but took the stage to find the arena on the edge of pitched battle. The Masters of Creative Situations, a Communist action group, chose the gig as the venue for a confrontation with the police, which necessarily spilled over onto the stage.”
= = =
from Dave Thompson, Your Pretty Face is Going to Hell: The Dangerous Glitter of David Bowie, Iggy Pop, and Lou Reed (NY: Backbeat Books, 2009), page 241.
RADICAL ARCHIVES NOTE: Googling this group doesn’t turn anything up; I am assuming they are autonomists by their name, which might also be rendered incorrectly. If you know anything about them, please post it in the comments.
Update December 2013: History is Made at Night has a good post about this.
radicalarchives 
This pic of a bootleg album of the concert says the groups were a mixture of ‘communists and fascists’ who called Reed a ‘dirty jew’. Sounds fairly improbable for Italy in 1975 but hey!
I was at this show as a 15 yr old American student living in Milan. It was my first concert and I remember it perfectly. There was virtually no security there. Those idiots behind the stage were throwing water balloons on the opening act. Then someone threw a a stone or hard object that hit the drummer in the head, triggering a two hour long fiasco with multiple people from the audience getting on stage and spouting communist party rhetoric. And they were “the good guys”. They wanted the show to go on. But the fascists behind the stage kept up with their antics. Finally, Lou Reed came out and said in shaky Italianthet if there was more trouble, the show was “fini”. He played two songs, opening with Sweet Jane and halted the show due to someone in the autonomists group throwing what looked like a towel or cloth onto the stage. There was some fighting and scuffles within isolated groups but I wouldn’t have characterized it as a “riot”. Still, it remains as one of the most memorable concert experiences of my life.