The Renton PD seemed to employ a "out of plain sight" approach.
Patrol vehicles strategically placed, a few uniform officers were near occassionally chatting with protestors, and patrol vehicles driving by the scene.
At one point a team of bicycle cops appeared out of no where rode through along side the protestors.
Each "side" seemed to be respectful of each other.
The approach of the Renton Police Department not antagonizing the protestors had it's intended effect--the protestors protested peacefully. In fact, when police vehicles flashed lights, or chirped sirens in support of the protestors, the protestors cheered in support back.
I have seen this so many times where police stay out of the way and everything goes smoothly. I think US police actually want a riot. They want to create violence.
More realistically, I think it is rarely this simple, on either side. I'm sure that usually there is a feedback loop of escalating responses.