Who can initiate an optional referendum at the national level?

Constitution of the Republic of Singapore

(adopted in 9 August 1965, last amended 2010)

http://www.wipo.int/wipolex/en/text.jsp?file_id=188429#LinkTarget_8238

Constitution Part II. (2)  A Billseeking to amend any provision in this Constitution shall not be passed byParliament unless it has been supported on Second and Third Readings by thevotes of not less than two-thirds of the total number of the elected Members ofParliament referred to in Article 39 (1) (a).

*(2A) Unless the President, acting in his discretion, otherwise directs theSpeaker in writing, a Bill seeking to amend —

...

 President may withhold assent to certainconstitutional amendments

†5A.

—(1) Subject to Part III, the President may, acting in his discretion, inwriting withhold his assent to any Bill seeking to amend this Constitution(other than a Bill referred to in Article 5(2A)), if the Bill or any provisiontherein provides, directly or indirectly, for the circumvention or curtailmentof the discretionary powers conferred upon the President by this Constitution.

(2) The President, acting in accordance with the advice of the Cabinet, maypursuant to Article 100 (whether before or after his assent has been withheldto a Bill under clause (1)), refer to a tribunal for its opinion the questionwhether the Bill or any provision therein provides, directly or indirectly, forthe circumvention or curtailment of the discretionary powers conferred upon thePresident by this Constitution; and where such a reference is made to thetribunal, Article 100 shall apply, with the necessary modifications, to thatreference.

(3) Where a reference is made to the tribunal and the tribunal is of theopinion that neither the Bill nor any provision therein provides, directly orindirectly, for the circumvention or curtailment of the discretionary powersconferred upon the President by this Constitution, the President shall bedeemed to have assented to the Bill on the day immediately after the day of thepronouncement of the opinion of the tribunal in open court.

(4) Where the tribunal is of the opinion that the Bill or any provisiontherein provides, directly or indirectly, for the circumvention or curtailmentof the discretionary powers conferred upon the President by this Constitution, andthe President either has withheld or withholds his assent to the Bill underclause (1), the Prime Minister may at any time direct that the Bill besubmitted to the electors for a national referendum.

(5) If the Bill referred to in clause (4) is supported at the nationalreferendum by not less than two-thirds of the total number of votes cast byelectors registered under the Parliamentary Elections Act (Cap. 218), thePresident shall be deemed to have assented to the Bill on the day immediatelyafter the publication in the Gazette of the results of the national referendum.

(6) For the purposes of this Article, where, on the expiration of 30 daysafter a Bill has been presented to the President for his assent, the Presidenthas neither signified the withholding of his assent to the Bill nor referredthe Bill to a tribunal pursuant to Article 100, the President shall be deemedto have assented to the Bill on the day immediately following the expiration ofthe said 30 days.