Police cards are bad, mean and sometimes brutal. For the most part they send demonstrants on the field to the prison, but some of them discards extremists.
Every player starts with 3 face down (you may look at these yourself) police cards on their board. These are the only Police cards a player get in the whole game! (except if they trade).
How to use
On your round, you can choose to spend 1 action to use 1 of your police cards.
First state which player you're using the police card on.
Secondly, choose which card you will use against that player.
Thirdly the chosen player suffers the effect of the card.
Since some of the police cards only affect players with demonstrants on the field, you should wait untill your target have that.
The best police card removes 1 extremist from the game, so have this in mind when playing the police cards.