"Polis çekildi duydum" = this is not a right sentence. But you can use like this : Polis çekildi diye duydum= i heard as policeman has step back. Polisin çekildiğini duydum= i heard that policeman has step back.

Polis çekildi duydum is correct if you have a comma after çekildi, and perhaps an excl mark after duydum - i.e Polis çekildi, duydum! So in english that would be Polis çekildi, duydum - The police has withdrawn, (yes), I heard it! (in a way you heard and saw them) Polisin çekildiğini duydum - I heard the police has withdrawn (in a way you were told so)

Polis çekildi duydum. - if u see, know and sure Polis’in çekildiğini duydum. - if u here somewhere but dont know and sure