kettle
n.— «It was obvious by now that people were not going anywhere until the two men were freed. So more reinforcements were brought in and surrounded the protest in what is known as a “kettle.” They soon managed to push people back into the car park and penned them in again, but the road remained blocked by police cars» —“Police Mar Peaceful Protest Against DRC Deportations in Solihull” Indymedia U.K. Apr. 13, 2007. (source: Double-Tongued Dictionary)