It shall be unlawful for any person, firm, or corporation to erect or install any sign, signpost advertisement, signpost, or other form of sign support, or any part thereof, upon or in such manner as to project wholly or in part over the right-of-way of any street or avenue (including but not limited to, any sidewalk or sidewalk area) or alley of the city, or to permit any such sign owned or leased by such person, firm, or corporation to remain, except as hereinafter provided.