"I promise you, this terrorist group [PKK] will be over after April 16. God willing, 'yes' will win [the referendum] and the FETO, PKK and Daesh will all be done with," he said at a Yes campaign rally in eastern Bingol province.
Yildirim said the ruling Justice and Development (AK) Party embraced all ethnicities in Turkey, and called on everyone to be united under the Yes campaign.
“No ethnic group living on this land is superior to another. All citizens living in this country including the Zazas, Kurds, Turks, Arabs, Roma people, Turkmen and others are all dear to our heart," he said.
The premier added that the proposed amendments to the constitution would further contribute to democracy.
"It will raise the standards of democracy and contribute to the culture of reconciliation," he said.
The constitutional changes have been discussed since Erdogan was voted president in August 2014. The 18-article bill was passed by parliament in January, with 339 votes in favor -- nine more than needed to put the proposal to a referendum.
The reforms would hand wide-ranging executive powers to the president and the post of prime minister would be abolished. The president would also be allowed to retain ties to a political party.
Other changes would see the minimum age for parliamentary candidates reduced to 18 and the number of deputies rise to 600. Simultaneous parliamentary and presidential elections for a five-year term would be held in November 2019 under the new constitution.