Let's imagine a situation. One dog's master set his pit bull on his neighbour. This dog bared teeth and did not allow the neighbour to get into his car. The dog frightened this man to death. The man threatened that he would go to the police and write a complaint [letter] about this event or that he would fill in a specific blank in the police station. What are idiomatic phrases that you would use, speaking about police? What would you say to this abuser who trains his dog in such a way?
Note, the situation is not so serious that you want call police immediately. You threaten that you'll make a complaint about this event in the police station when you'll get off work, say, tonight.