Section 2. A town may elect a board of park commissioners, consisting of three or five persons and prescribe their terms of office, or the members of a town planning board or department of public works, or road commissioners may if so authorized by vote of the town act as park commissioners therein. In a town which has not elected a board of park commissioners or has not authorized the town planning board so to act, the selectmen shall act as such board of park commissioners and have authority to approve or disapprove the acquisition of lands by eminent domain as set forth in section seventy-nine of chapter ninety-two and shall have all other powers and duties of a park commission. The mayor of a city may, with the approval of the city council, appoint a board of park commissioners for said city, consisting of five persons, who shall hold office for terms of one, two, three, four and five years respectively from the first Monday in May next following such appointment, or until their successors are qualified; and thereafter the mayor shall annually, before the first Monday in May, with like approval appoint one such commissioner for a term of five years from said first Monday in May. No member of the city council, clerk or treasurer of such city or town shall be such commissioner. In cities a vacancy in such board shall be filled in like manner for the residue of the unexpired term. A commissioner may be removed by a vote of two thirds of the voters of a town, or by a vote of two thirds of all the members of a city council. Such commissioners shall serve without compensation.