I was caught in some of the rioting that happened in Rochester, NY during the 60s - it was the rioters then too. They were hell-bent on rioting and looting whether cops were there or not - the cops added to the tension because they were a sign of unwanted authority and an impediment to what the rioters wanted to do.
I was in Miami and I think the most problems were caused by the cops. But that was Miami.