Who can initiate an optional referendum at the national level?

Constitution (2010)

http://www.kenyalaw.org/klr/index.php?id=741

Article 255

 (3) An amendment to this Constitution thatdoes not relate to a matter mentioned in clause (1) shall be enacted either—

   (a) by Parliament, inaccordance with Article 256; or

   (b) by the people andParliament, in accordance with Article 257.

Article 256

 (5) If a Bill to amend this Constitutionproposes an amendment relating to a matter mentioned in Article 255 (1)—

   (a) the President shall, before assenting to the Bill,request the Independent Electoral and Boundaries Commission to conduct, withinninety days, a national referendum for approval of the Bill; and

Article 257

(1)  An amendment to this Constitution may be proposed by a popular initiativesigned by at least one million registered voters.