At the election at which this charter shall be submitted there shall be elected six councilmen and a mayor at large whose terms of office shall begin upon taking effect of this charter. The candidate for city councilman who receives the highest number of votes at large shall be elected councilman for the district for which he has filed his petition. The mayor and councilmen shall serve until the Monday following the next regular city election, or as provided in Section 6. 2(b) above.