To generalize this beyond just the noun "police", when you form a question tag for a statement using some form of "to be", you echo the verb form that was used in the actual question, negated, plus a matching pronoun. Since you said "the police are here", your tag question also uses are (with the appropriate negation). And since are is a plural form, your question tag must also take a plural pronoun.
So any time you say "X are Y", the appropriate question tag is "aren't they?".
For "X is Y", it's one of "isn't he?", "isn't she?", or "isn't it?", depending on the gender of X.
For "X aren't Y", the question tag is "are they?"
For "You are Y", it's "aren't you?" and for "You aren't Y", it's "are you?".
For "I'm not Y", the question tag is "am I?".
The only interesting case is when you are question-tagging a positive first-person statement:
"I am going, ____ __?"
According to the rule I just laid out, you should be saying "amn't I?", but that is no longer accepted English. (You may encounter it in older works but I sincerely doubt you will ever hear anyone use it as part of their standard conversational style.) Instead, the normal tag is "aren't I?". However, you may also hear "am I not?".