The maneuver was a law enforcement tactic called kettling. The police encircle protesters so that they have no way to exit from a park, city block or other public space, and then charge in and make arrests.
For the next 20 minutes in Downtown Brooklyn, officers swinging batons turned a demonstration that had been largely peaceful into a scene of chaos.
The kettling operations carried out by the police department after curfew have become among the most unsettling symbols of its use of force against peaceful protests, and have touched off a fierce backlash against Mayor Bill de Blasio and the police commissioner, Dermot F. Shea.