Learn English – How to address more than one person

The official term is police officer, so officer seems to be the best term, neutral and respectful. It also seems valid regardless of their rank, gender, etc.

Excuse me officer, may I ask why my car was stopped?
Excuse me officer, I just wanted to tell you that you have a headlight out.

Answers to this question at ELU also suggest sir and ma'am (not madam, thanks @tchrist) as good alternative.
Also, UK English has another word, constable, which is a bit too official, but quite acceptable.

And don't try slang words! In some countries, it may lead you straight to jail.

P.S. In some countries, a police officer holds a highly respected position, and rank does matter. So if you are traveling and you can read insignia of ranks and you know what you are doing, you may call them by rank to show your highest respect:

Excuse me lieutenant, ...

While it's grammatically correct to use an explicit subject with the imperative mood (e.g. "Somebody help!", "Nobody move!", "Don't everybody talk at once!", "Don't you dare go in there!"), it's somewhat informal, not very common in professional writing. In professional writing, the best solution is to write:

Managers, please feel free to forward this request to your team if you believe they can give helpful feedback.

with a comma after "managers". Here the word "managers" indicates who the sentence is addressed to (compare "Hi, <person's name>, how are you?"), but the bulk of the sentence uses an implicit "you" (as is normal in the imperative mood).