Section 117. A caucus of the voters, or of a specified portion thereof in a ward of a city, or in a town, may be called and held for the nomination of candidates to be voted for at any city election, or at any election of town officers for which official ballots are used, or for the selection of delegates to a convention, or for the appointment of a committee. The proceedings of such caucuses shall be invalid unless at least twenty-five voters participate and vote therein. Except as provided in this section and in section one hundred and twenty-one, no caucus or meeting other than those of political parties shall be entitled to nominate a candidate whose name shall be placed on the official ballot, or to select delegates to a political convention for the nomination of a candidate whose name shall be placed on such ballot.