‘Kettling’ of Peaceful Protesters Shows Aggressive Shift by N.Y. Police

“We need healing,” Mr. Shea said. “We need dialogue. We need peace.”

For many protesters, however, the hard-nosed tactics the police have employed to shut down marches after curfew have only exacerbated the violence.

Axel Hernandez, 30, was protesting at Cadman Plaza on Wednesday night when officers rushed into the crowd. Mr. Hernandez, who had marched several times this week, said that up until that point it had been one of the calmest demonstrations he had attended.

“That was the most peaceful, no bottles thrown, no anything,” he said. “The next thing I know, police rush in, with batons, and started moving people, and start hitting people.”

Experts on crowd control say kettling is a technique the police have used for decades, not just in New York City, but around the world, including Northern Ireland. In theory, officers surround protesters, cutting off exits until they tire, then let them disperse in small groups.

But because demonstrators have nowhere to go, the maneuver often ends with a charge and mass arrests. Since the city put a curfew in place this week, the police have used the technique in Brooklyn, Manhattan, and the Bronx.

“Kettling is basically when you take the crowd and drive it into a box, which is a great idea if you’re wanting to capture people,” said Dennis Kenney, a criminal justice professor at John Jay College of Criminal Justice. “It’s generally a way to greatly increase the likelihood of conflict.”