Looks like the police are employing kettling.
https://en.wikipedia.org/wiki/Kettling
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.