Are policemen only officers?
For example, in Belarus street patrols (they are going by foot) have no officers. Only road patrols have or consist of officers.

Using of "Гражданин/товарищ милиционер/сержант/капитан, etc." is obsolete and "канцелярщина" (how to say it in English?). Usually if you are asking something at the street you should use something like "Здравствуйте/Извините, не могли бы Вы сказать ..." (the same way as with other people ). And when militiaman starts the conversation he introduces himself, something like "Здравствуйте, я - лейтенант милиции, Иванов Иван Иванович, ваш участковый", and then you can use no name or "Иван Иванович", when you speak with him.

And when he doesn't introduce, even he is in uniform, you should consider that he is not a militiaman.
For example, real story. If you see any person in uniform trying to catch you or something similar without introduction and showing documents you can resist. So, the real story: My friend, he is a boxer, was walking through the street and suddenly he heard "Catch him !", two persons were running towards him. And one of them tried to catch him. Just tried... Than other introduced, and asked to forgive his mate, because they are catching prisoner and his mate hurried. (If my friend sued than militiaman could lose his work).

My intuition says to me, that I made terrible amount of mistakes. So, please, correct them.