Who can initiate an optional referendum at the national level?

Spanish Constitution 1978 http://www.tribunalconstitucional.es/es/constitucion/Paginas/ConstitucionIngles.aspx

Article 92

1. Political decisions of special importance may be submittedto all citizens in a consultative referendum.

2. The referendum shall be called by the King at theproposal of the President of the Government, following authorisation by theCongress of Deputies. [...]

Consultive Referendum (optionalreferendum) requires the authorization given by the absolute majority of thelower house (Congreso de los Diputados) to the previous request of thePrime Minister. This request has to contain the exact wording ofthe referendum question.