Who can initiate an optional referendum at the national level?

Serbia, Constitution 2006, accessed 20 December 2021

ARTICLE 108. REFERENDUM

Upon the request of the majority of all deputies or at least 100,000 voters, the National Assembly shall call the referendum on issues falling within its competence, in accordance with the Constitution and Law.

The subject of the referendum may not include duties deriving from international contracts, laws pertaining to human and minority rights and freedoms, fiscal and other financial laws, the budget and financial statement, introduction of the state of emergency and amnesty, as well as issues pertaining to election competences of the National Assembly.

Article 203: Proposal to amend the Constitution and adoption of the amendment to the Constitution

A proposal to amend the Constitution may be submitted by at least one third of the total number of deputies, the President of the Republic, the Government and at least 150,000 voters. [...]

The National Assembly shall adopt an act on amending the Constitution by a two-third majority of the total number of deputies and may decide to have it endorsed in the republic referendum by the citizens.

The National Assembly shall be obliged to put forward the act on amending the Constitution in the republic referendum to have it endorsed, in cases when the amendment of the Constitution pertains to the preamble of the Constitution, principles of the Constitution, human and minority rights and freedoms, the system of authority, proclamation the state of war and emergency, derogation from human and minority rights in the state of emergency or war or the proceedings of amending the Constitution.