Who can initiate an optional referendum at the national level?

1987 CONSTITUTION OF THE REPUBLIC OF THE PHILIPPINES

http://www.chanrobles.com/philsupremelaw1.htm

Article XVII

AMENDMENTS ANDREVISIONS

Section 1. Anyamendment to, or revision of, this Constitution may be proposed by:

    The Congress, upon a vote of three-fourthsof all its Members; or

    A constitutional convention.

Sec. 2. Amendmentsto this Constitution may likewise be directly proposed by the people throughinitiative upon a petition of at least twelve per centum of the total number ofregistered voters, of which every legislative district must be represented byat least three per centum of the registered votes therein. No amendment underthis section shall be authorized within five years following the ratificationof this Constitution nor oftener than once every five years thereafter.

The Congress shall provide for the implementation ofthe exercise of this right.

Sec. 3. The Congress may, by a vote of two-thirds ofall its Members, call a constitutional convention, or by a majority vote of allits Members, submit to the electorate the question of calling such aconvention.

RepublicAct No. 6735 'The Initiative and Referendum Act'

Section 1. Title. — This Act shall be known as 'The Initiativeand Referendum Act.'

Sec. 2. Statement of Policy. — The power of the people under asystem of initiative and referendum to directly propose, enact, approve orreject, in whole or in part, the Constitution, laws, ordinances, or resolutionspassed by any legislative body upon compliance with the requirements of thisAct is hereby affirmed, recognized and guaranteed.

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Sec.  4. Who may exercise. — The power of initiative andreferendum may be exercised by all registered voters of the country, autonomousregions, provinces, cities, municipalities and barangays