Lying on the pavement of a parking lot, Cyprian Luke’s face reddened as an officer’s hand clamped his throat.

“Put your hands behind your back!” the officer shouted, then punched Mr. Luke twice in the temple.

By the time Mr. Luke, 19, was dragged to a police cruiser, his mouth and face were bloody and raw.

Footage of the Sunday incident in Dover, N.J., was the latest online posting to prompt cries of police brutality and racism, exposing a rift in another community that feels terrorized by its own authorities.