Who can initiate an optional referendum at the national level?

Finland
THE FINNISH CONSTITUTION
(Adopted1999; last amended 2011)
http://www.finlex.fi/fi/laki/kaannokset/1999/en19990731.pdf
Section 53 – Referendum and citizens’ initiative (new title, 1112/2011,entry into force 1.3.2012)
The decision to organise a consultative referendum ismade by an Act, whichshall contain provisions on the time of the referendum and on the choices to bepresented to the voters.
PARLIAMENT’S RULES OF PROCEDURE
(Adopted 1999; last amended 2012)
http://web.eduskunta.fi/dman/Document.phx?documentId=lp13613093258049&cmd=download
Section 61 —Mode ofvoting
Avote shall be taken by the voting apparatus, by standing up or by open ballot.
TheSpeaker shall announce the mode of voting. There shall be no debate on the modeof voting.
Avote shall be taken by the voting apparatus, regardless of whether a vote has alreadybeen taken by standing up, if the Speaker deems this necessary, if the vote by standing up has in the opinion of theSpeaker not yielded a clear result or ifa Representative so requests.
Avote shall be taken by open ballot if the voting apparatus is out of order, if thevote by using the voting apparatus has in the opinion of the Speaker not yieldeda clear result or if thereis a tie. (…)
Section 62 —Vote byopen ballot
National referenda can be initiated by an Act of Parliament. As implied bySections 61 and 62 of Parliament’s Rules of Procedure, Acts can only be passedby a majority.