The memorable music score for this scene also should earn it points as it added to the memorability of such a shocking and intense moment.
One thing to note is that Hitchcock often has policemen in his films portrayed as imposing and cruel; due to a fear of the police he'd had since he was three years old. His father had sent him over to the police station with a note and gave it to the one at the desk. The policeman looked it over, then grabbed the young Alfred and stuck him in a jail cell and left him there for several minutes. Turned out his father had arranged all this(the note had instructed the cop to lock Alfred in the jail cell for several minutes) because he was trying to scare Alfred into behaving himself by showing the consequences of what would happen if he were not to, but instead had traumatized him.