Legal provisions for optional referendums at national level

Italy
Constitution of 1947 with Amendmentsthrough 2012
https://www.constituteproject.org/constitution/Italy_2012.pdf
Initiation of general legislation
Art 71
Legislation may be introduced by theGovernment, by a Member of Parliament and by those entities and bodies soempowered by constitutional amendment law.
The people may initiate legislationby proposing a bill drawn up in sections and signed by at least fifty-thousandvoters.
Referenda and initiative
Art 75
A general referendum may be held torepeal, in whole or in part, a law or a measure
having the force of law, when sorequested by five hundred thousand voters or five
Regional Councils.
No referendum may be held on a lawregulating taxes, the budget, amnesty or pardon, or a law ratifying aninternational treaty.
Any citizen entitled to vote for theChamber of deputies has the right to vote in a referendum.
The referendum shall be consideredto have been carried if the majority of those eligible has voted and a majorityof valid votes has been achieved. The procedures for conducting a referendumshall be established by law.
Art 87
[…]
The President shall:
[…]
- call a general referendum in thecases provided for by the Constitution;
[…]
Regional and local referenda
Art 123
Each Region shall have a statutewhich, in compliance with the Constitution, shall lay down the form ofgovernment and basic principles for the organisation of the Region and theconduct of its business. The statute shall regulate the right to initiate legislationand promote referenda on the laws and administrative measures of the
Region as well as the publication oflaws and of regional regulations.
Regional statutes are adopted andamended by the Regional Council with a law approved by an absolute majority ofits members, with two subsequent deliberations at an interval of not less thantwo months. This law does not require the approval of the Governmentcommissioner. The Government of the Republic may submit the constitutionallegitimacy of the regional statutes to the Constitutional Court within thirtydays of their publication.
The statute is submitted to popularreferendum if one-fiftieth of the electors of the Region or one-fifth of themembers of the Regional Council so request within three months from itspublication. The statute that is submitted to referendum is not promulgated ifit is not approved by the majority of valid votes.
Art 132
By a constitutional law, afterconsultation with the Regional Councils, a merger between existing Regions orthe creation of new Regions having a minimum of one million inhabitants may beagreed, when such request has been made by a number of Municipal Councilsrepresenting not less than one-third of the populations involved, and therequest has been approved by referendum by a majority of said populations.
The Provinces and Municipalitieswhich request to be detached from a Region and incorporated in another may beallowed to do so, following a referendum and a law of the Republic, whichobtains the majority of the populations of the Province or Provinces and of theMunicipality or Municipalities concerned, and after having heard the RegionalCouncils.
Art 138
Laws amending the Constitution andother constitutional laws shall be adopted by each House after two successivedebates at intervals of not less than three months, and shall be approved by anabsolute majority of the members of each House in the second voting.
Said laws are submitted to a popularreferendum when, within three months of their publication, such request is madeby one-fifth of the members of a House or five hundred thousand voters or fiveRegional Councils. The law submitted to referendum shall not be promulgated ifnot approved by a majority of valid votes.
A referendum shall not be held ifthe law has been approved in the second voting by each of the Houses by amajority of two-thirds of the members.