Be it further enacted, That the electors shall have power to approve or reject at the polls any ordinance passed by the council, or submitted by the council to a vote of the electors, such power been known as the referendum. Ordinances submitted to the council by initiative petition and passed by the council without change shall be subject to the referendum in the same manner as other ordinances. Within 14 days after the enactment by the council of any ordinance which is subject to a referendum, a petition signed by qualified electors of the city equal in number to at least 25 per centum of the registered voters at the last preceding regular municipal election may be filed with the city clerk requesting that any such ordinance be either repealed or submitted to a vote of the electors.