It shall be lawful for any city now having or which may hereafter have five hundred thousand or more inhabitants to provide by ordinance for the establishment of building lines on any public street. Such building line shall be established by the same procedure as that provided by law in such city for the acquiring of land for the opening of streets. After the establishment of any such line no building or other structure shall be erected, reconstructed or substantially repaired and no new building or other structure or part thereof shall be reerected within said lines so established.