Who can initiate an optional referendum at the national level?

Constitution of 1987 (last amended in 1992)

ARTICLE 71 (2). The National Assembly shall have the power to decide by 2/3 majority on the organization of a People’s Assembly or a plebiscite in those cases deemed necessary by the National Assembly, without prejudice to the provisions of Article 179, paragraph 2.

ARTICLE 83 (3). A majority of at least 2/3 of the constitutional number of members of the National Assembly shall be required for decisions concerning:

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f. The organization of a plebiscite.