If you're truly concerned about needing someone with a gun (the main reason you call the police in the United States of America is someone going buck wild with a machine gun), you can look up the phone number of your nearest police station in many places. The phone number will be something like XXX-YYY-Z911 where X is the area code and Y is the NPA (usually specific to a township or section of the city).

For anything else, like a sexual assault, go directly to the station and hope and/or pray they are willing to do more than provide you with a reference number then go back to doing nothing or worse than nothing.

(I'm sorry if the above sounds harsh, but I used to joke in my open mic that even Timothy McVeigh would pull over for the highway patrol -- we have forgotten why we set up these systems, and brought fascism upon our homeland, and we must not be silent anymore, no matter what the context, for silence will apparently be taken as consent of the governed.)

- Greg.