The people of the City reserve the power to approve or reject at the polls any legislation enacted by the Council which is subject to the initiative process under this Charter. Within thirty (30) days after the final adoption or publication, whichever date is later, of any ordinance which is subject to referendum, a petition, signed by registered voters of the city equal in number to at least twenty (20) percent of the number of registered voters residing in the City at the time of the last regular City election, may be filed with the City Secretary requesting that any such ordinance be either repealed or submitted to a vote of the people. When such a petition has been certified as sufficient by the City Secretary, the ordinance so specified in the petition shall not go into effect, or further action thereunder shall be suspended if it shall have gone into effect, until and unless it is approved by the voters as herein provided.