Now, both police and protesters believe that a steady stream of new videos revealing the confrontations has brought about a turning point and a question: Are these the tactics police in the United States should be using?

These were scenes not seen so widely in the United States in decades, scenes that police training, recruitment and reform were intended to prevent: officers striking unarmed protesters, in the heart of American cities, carrying out orders.

Even among police leaders, there is a sense that these incidents — and, in some cases, misleading official accounts given before the videos emerged — could do lasting damage to the image of American police, most of whom have never been involved in violent encounters with anyone.

“We certainly, as a profession, have been diminished by events that have been witnessed on video over the course of the last couple of weeks,” said Jim Pasco, executive director of the national Fraternal Order of Police, a labor union. “And the burden is on us to reestablish and build a relationship with the community. We can’t do our job without the community. So the burden is on us going forward.”