The tactics used to control riots in the past were simple -- they were based on the fact that the police were almost always better-armed than the rioters. The tactics they used basically consisted of forming a line and charging into the crowd. Today, the police are still well-armed, but tactics have advanced significantly in hopes of preventing injuries.
When a riot is in full swing, police will deploy in a square formation with a command team at the center. The command team is protected on all four sides by echelons of troops deployed in groups of 10 or 12 officers. There is also an arrest team at the center of the square.
This tactical unit is very mobile and able to adapt on the fly to changes in the situation. If a threat suddenly appears behind or to one side of the unit, then the echelon facing that direction is designated the front of the unit. The entire team can then change the direction it's facing without a lot of maneuvering. Also, the echelons can cover each other when the team moves to take advanced positions. If the unit is under attack, the whole team does not move together: One echelon moves while the others provide covering fire or an actual physical screen (with riot shields). Then another echelon moves up into position.
The echelon is not meant to be an impenetrable wall of cop. In fact, the riot squad often leaves an escape route to let rioters run past the squad. The officers can adopt a passive position, in which they spread out and leave several yards between each officer. The crowd can then easily filter through them. If a particularly violent group moves toward the officers or they spot specific suspects they want to arrest, they can quickly close the gaps and form a tight line.
As the unit moves forward into a crowd, it will prod and push at anyone who doesn't respond to requests to move away by the time the front echelon reaches them. If they still refuse to move, the unit continues moving forward, but the front echelon opens up and passes around the protestors. Once the protestors are inside the square, the unit stops, the front echelon reforms and the arrest team processes the rioters. When they're done, the unit can continue moving.
In the next section, we'll see what equipment crowd-control units use to do their job.