Who can initiate an optional referendum at the national level?

Constitution of1947 with Amendments through 2012

https://www.constituteproject.org/constitution/Italy_2012.pdf

Initiationof general legislation

Art71

Legislationmay be introduced by the Government, by a Member of Parliament and by thoseentities and bodies so empowered by constitutional amendment law.

Thepeople may initiate legislation by proposing a bill drawn up in sections andsigned by at least fifty-thousand voters.

Referendaand initiative

Art75

Ageneral referendum may be held to repeal, in whole or in part, a law or ameasure having the force of law, when so requested by five hundred thousandvoters or five Regional Councils.

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Art87

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ThePresident shall:

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-call a general referendum in the cases provided for by the Constitution;

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Art138

Lawsamending the Constitution and other constitutional laws shall be adopted byeach House after two successive debates at intervals of not less than threemonths, and shall be approved by an absolute majority of the members of eachHouse in the second voting.

Saidlaws are submitted to a popular referendum when, within three months of theirpublication, such request is made by one-fifth of the members of a House orfive hundred thousand voters or five Regional Councils. The law submitted toreferendum shall not be promulgated if not approved by a majority of validvotes.

Areferendum shall not be held if the law has been approved in the second votingby each of the Houses by a majority of two-thirds of the members.