Section 23 - Powers hereby granted, how to be exercised1. The powers granted by this act are to be exercised by the officer, officers or official body vested with such powers by any other provision of law or ordinance (subject to amendment or repeal of any such ordinance) and in the manner and subject to the conditions prescribed by law or ordinance (subject to amendment or repeal of any such ordinance), but no provision of any special or local law shall operate to defeat or limit in extent the grant of powers contained in this act; and any provision of any special or local law which in any city operates, in terms or in effect, to prevent the exercise or limit the extent of any power granted by this article, shall be superseded. Where any such provision of special or local law is superseded under the provisions of this subdivision, such power, freed from the limitations imposed by such provision, shall be exercised by the same officer, officers or official body that would be vested with the same under the provisions of this subdivision, if such provision had not been superseded, but the exercise thereof shall be subject to the limitations provided for in subdivision two of this section.2. In the absence of any provision of law or ordinance determining by whom or in what manner or subject to what conditions any power granted by this act shall be exercised, the common council or board of aldermen or corresponding legislative body of the city shall, subject to the provisions of this section, have power by ordinance to determine by whom and in what manner and subject to what conditions said power shall be exercised. The exercise by any city of any power granted by this article not now vested in such city or now vested in such city subject to provisions which are superseded by the provisions of subdivision one of this section, shall be subject to the following limitations: b. No sale or lease of city real estate or of any franchise belonging to or under the control of the city shall be made or authorized except by vote of three-fourths of all the members of the common council or corresponding legislative body of the city. In case of a proposed sale or lease of real estate or of a franchise, the ordinance must provide for a disposition of the same at public auction to the highest bidder, under proper regulations as to the giving of security and after public notice to be published at least once each week for three weeks in the official paper or papers. A sale or lease of real estate or a franchise shall not be valid or take effect unless made as aforesaid and subsequently approved by a resolution of the board of estimate and apportionment in any city having such a board, and also approved by the mayor. No franchise shall be granted or be operated for a period longer than fifty years. The common council or corresponding legislative body of the city may, however, grant to the owner or lessees of an existing franchise, under which operations are being actually carried on, such additional rights or extensions in the street or streets in which the said franchise exists, upon such terms as the interests of the city may require, with or without any advertisement, as the common council may determine, provided, however, that no such grant shall be operative unless approved by the board of estimate and apportionment in any city having such a board, and also by the mayor. In any city the question whether any proposed sale or lease of city real estate or of any franchise belonging to or under the control of the city shall be approved shall, upon a demand being filed, as hereinafter provided, be submitted to the voters of such city at a general or special election, after public notice to be published at least once each week for three weeks in the official paper or papers. Such demand shall be subscribed and acknowledged by voters of the city equal in number to at least ten per centum of the total number of votes cast in such city at the last preceding general election and shall be filed in the office of the clerk of such city before the adoption of an ordinance or resolution making or authorizing such sale or lease. If such demand is filed, as aforesaid, such sale or lease of real estate or such franchise shall not take effect unless in addition to the foregoing requirements a majority of the electors voting thereon at such election shall vote in the affirmative. The foregoing limitations shall not apply to the exercise by any city of any power now vested in it, where the existing provisions of law determining by whom or in what manner or subject to what conditions such power shall be exercised are not superseded by the provisions of subdivision one of this section; but in such case the exercise of such power shall be subject only to such existing provisions of law, and shall not be limited or restricted by any provision of this section.