What is the sense of the word "kettle" here?

I've just come across the word "kettle" in a news article used in a sense I've not heard it before.

"Police have formed 'a kettle' at Millenium Bridge, according to some of the protesters. They claim it is to prevent people sleeping on the streets. Others suggest police are simply moving people on. "

It doesn't seem to fit any of the dictionary definitions of the word. Please can someone explain.

Solution 1:

It's a strategic formation. It's normally kettling, and I suppose "kettle" is the noun form of it. It's described thus:

Kettling (also known as containment or corralling) 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.

The police formation looks like a "kettle".

The link I gave has several instances of kettling described for further detail.

Solution 2:

Kettling (also known as containment or corralling) 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 are left only one choice of exit, determined by the police, or are completely prevented from leaving.

Wikipedia

Solution 3:

According to the BBC the term is derived from the German military usage of the word Kessel (cauldron) meaning an encircled military force, for example in the Keil und Kessel tactic used extensively against the Soviet Army in WWII.

The word derives from the German word "kessel" - literally a cauldron, or kettle - to describe an encircled army about to be annihilated by a superior force. For soldiers within the kettle the situation would soon become unbearable hot.