Whenever there shall be presented to the City Council a petition or petitions, signed by twenty (20) per cent of the registered and qualified legal voters of said city, demanding that an ordinance passed by the council, and set forth in said petition or petitions, be submitted to a vote of the qualified electors of the city, it shall be the duty of the council to submit such proposed ordinance to a vote of the qualified electors of said city at the next election held thereafter by said city. The signatures to said petition need not all be appended to one paper, but each signer shall add to his signature his place of residence, giving the street and number. All signatures must be made in ink or with indelible pencil. One of the signers of said petition shall make oath before a duly qualified officer that the statements therein are true and that each signature to said petition is a genuine signature of the person whose name it purports to be and was signed in his presence by such person; provided, that any petition intended to require a submission to referendum of an ordinance enacted by the City Council shall be presented to the council within thirty (30) days after the enactment of such ordinance, and not thereafter. The City Council may, if in its judgment the public interest so demands, provide for such referendum election to be held at an earlier date than that of the next ensuing election, in which event the said date shall be fixed by the council.