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 being known as the referendum, except in cases of bond ordinances and ordinances making the annual tax levy or annexing or de-annexing additional territory. 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 twenty calendar 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 twenty-five percent of the number of votes cast 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last preceding regular municipal election may be filed with the City Clerk requesting that any such ordinance be either repealed or submitted to a vote of the electors.