"What do we do if we're being kettled?" According to Wikipedia:

Kettling (also known as containment or corralling)[1] is a police tactic for controlling large crowds during demonstrations or protests. It involves the formation of large cordons of police officers who then move to contain a crowd within a limited area. Protesters either leave through an exit controlled by the police or are contained, prevented from leaving, and arrested.

First I've heard this term too.