Who can initiate an optional referendum at the national level?

Constitution of Jamaica

(Adopted 1962, last amended 2011)

http://pdba.georgetown.edu/Constitutions/Jamaica/jam62.html

49.1: Subject to the provisions of this section Parliament may byAct of Parliament passed by both Houses alter any of the provisions of thisConstitution or (in so far as it forms part of the law of Jamaica) any of theprovisions of the Jamaica Independence Act, 1962.

49.4: A Bill for an Act of Parliament under this section shall notbe deemed to be passed in either House unless at the final vote thereon it issupported-

1. in the case of a Bill which alters any of the provisions specified in subsection (2) or subsection (3) of this section by the votes of not less than two-thirds of all the members of that House, or
2. in any other case by the votes of a majority of all the members of that House.

For an optional referendum to take place, a bill to amend any of the provisions specified in Sections 49.2 and 49.3 of the Constitution must pass the house, as specified in 49.4. If the bill is rejected by the Senate, the House can call a referendum.