The town council of all incorporated towns of this Commonwealth shall consist of six councilmen and a mayor who shall be president of the town council. At the municipal election in the year one thousand nine hundred and twenty-nine, six councilmen shall be elected at large by the voters in each such town, three for terms of two years each, and three for terms of four years each. Biennially thereafter, at each municipal election, three councilmen shall be elected for terms of four years each. At the municipal election in the year one thousand nine hundred and twenty-nine, and every four years thereafter, a mayor shall be elected by the voters for a term of four years. The term of the councilmen and mayor shall commence on the first Monday of January succeeding their election. Vacancies in the office of town council and mayor shall be filled in the manner now provided by law.