Except as prohibited by general law, the qualified electors shall have power to approve or reject at the polls any ordinance passed by Council, or any ordinance submitted by Council to vote of the qualified electors, such power being known as the referendum. Ordinances submitted to Council by initiative petition and passed by Council without change shall be subject to the referendum in the same manner as other ordinances. Within thirty days after the enactment by Council of any ordinance which is subject to a referendum, a petition signed by qualified electors equal in number to at least ten per centum of the registered voters at the last general municipal election may be filed with the City Clerk requesting that such ordinance be either repealed or submitted to a vote of the qualified electors.