The video shows a crowd of people holding large wooden banners slowly walking into a line of police bikes. It doesn't seem like a "violent struggle" to me, but a slow, determined plodding into police lines, using the wooden banners as plows to try and force the police out of the way.
Among the group pepper sprayed were 10 Portland high school students who were at the event with a student organization to protest public school budget cuts. Halley Steiner, a 17-year-old senior at Grant High, said she was holding a large wooden banner saying "No More Cuts" when they encountered the line of police. According to Steiner, the officers said, "Don't move forward or we will resist you." She doesn't remember hearing any warning about pepper spray or use of chemicals specifically, but before she moved, officers started pepper spraying the crowd. "It was the most painful thing I've ever experienced," says Steiner, who fell to the ground after a few seconds. "I was completely blind for 30 minutes, totally unable to even begin to open my eyes. The fact that we were students attempting to peacefully protest budget cuts to education and were met with such unrestrained police violence is revolting and unbelievable."
Update 2:30pm—A couple people have pointed out that one of the officers doing the pepper spraying was Sgt. Kyle Nice, who is only recently back on patrol duty after two police investigations into an road-rage encounter and his involvement in the death of James Chasse.