The national guard was there. Not sure if they did much of anything to keep the various factions separated though.
It is really hard to interpret that picture without knowing when it was taken.
I believe that is the corner of 4th &Main. Main runs parallel to Water St. and is bricked over in this area. It serves as the hub of a pedestrian mall with shops.
From what I understand, after the car ramming, rioting broke out on Main St, and at this point, the cops came in and cleared out the area, My guess is that police line was formed after the homicide occurred, not before.