Any time there is some sort of a large protest rally, some opportunists and anarchists have to spoil the cause.
The Mehserle trial in Los Angeles returned an "involuntary manslaughter w/ gun enhancement" guilty verdict. So what happens in Oakland? A peaceful rally starting a 6PM allowed people to get out of downtown Oakland if they want to... And the supports and anarchists and looters to gather about. Once it gets dark (9PM) the mess started.
The threat is from all over. The anarchists are instigating incidents by cheering on and starting attacks on police. Oakland PD exercised considerable restraint when they retreated one block after another at about 7PM to establish perimeter around the area. However, when it's dark, the police are concentrating on the main group, trying to contain them to downtown, and picking up the instigators / anarchists whenever possible. Then the bands of looters started to appear... Groups of 6-10 wearing hooded sweatshirts (known as "hoodies") with the hood up rushed shoe stores, phone stores.. Nothing too expensive to require extensive alarm, but contains something desirable. TV cameras covered a Foot Locker getting looted, and the damn looters are brave enough to rush in EVEN WHEN SURROUNDED BY CAMERAS. One even got his hood up as he ran in.
Volunteers in downtown, armed with just cellphones and their voice, yelled at potential looters, most of whom when on to search for unobserved shops to loot. And other OPD units are on hand chasing down looters.
And the tally says it all: more than half arrested are NOT Oakland residents.