Legal provisions for optional referendums at national level

Spain
Answer
Yes
Source
Article 92 of Spanish Constitution.
http://www.tribunalconstitucional.es/es/constitucion/Paginas/ConstitucionIngles.aspx#I23
Article 92 of Spanish Constitution.
http://www.tribunalconstitucional.es/es/constitucion/Paginas/ConstitucionIngles.aspx#I23