In Minneapolis, videos posted online showed police officers yelling, “Light ’em up!” before firing paint projectiles at residents as they stood on a front porch of a home. No one in the group appeared to be seriously injured.

Edward Maguire, a professor at Arizona State University who recently published a guidebook on police crowd-control procedures, said this incident was especially egregious because officers fired potentially dangerous rounds at people who posed no threat.

“Everything that police do in these types of situations should be aimed at de-escalation, and that is a really, really stunning example of escalation,” he said. “You cannot be shooting projectiles at human beings, unless you have a really good reason to do so.”

Watching the events of Saturday night, Maguire said he felt as if police chiefs across America had read his guidebook to crowd control — and decided to do the opposite. “I’m just seeing examples all over the country right now of bad policing,” he said. “Poorly conceived strategies for how to handle protests.”