Except as provided under section 590.750, the police commissioner of the said cities shall have power to regulate and license all private watchmen, private detectives and private policemen, serving or acting as such in said cities, and no person shall act as such private watchman, private detective or private policeman in said cities without first having obtained the written license of the president or acting president of said police commissioners of the said cities, under pain of being guilty of a misdemeanor.