Erdogan served three consecutive terms as prime minister as head of his Islamic-rooted Justice and Development Party, before becoming Turkey's first directly elected president in 2014.
"Now that Turkey has obviously taken the wrong path from a European perspective, we need to re-evaluate our relationship, and specifically that means that the EU accession talks with Turkey that we're still having can no longer be continued". The country has already put a bar on the flow of migrants, mainly refugees from war torn countries of Syria and Iraq.
Some 55 million people are eligible to vote at 167,140 polling stations which opened at 7 a.m. (0400 GMT) in the east and 8 a.m.in the rest of Turkey.
More than 55 million people in this country of about 80 million were registered to vote and more than 1.3 million Turkish voters cast their ballots overseas.
Istanbul resident Husnu Yahsi, 61, also said he was voting "no".
"I don't think one-man rule is such a scary thing".
Erdogan has promised the new presidential system will herald a period of stability and prosperity for Turkey, a country that has suffered several coups in the past few decades.
The referendum has bitterly divided the nation. The changes would come into effect with the next general election, scheduled for 2019. Erdogan survived a coup attempt last July, which he has blamed on his former ally and current nemesis Fethullah Gulen, an Islamic cleric living in the United States. This would not make Turkey stronger or better as they claim. He said voting was a response to those forces that threaten the country.More news: Pogba: We Want To Beat Chelsea
Deputy Prime Minister Veysi Kaynak said the "Yes" votes were lower than expected.
"Whatever the outcome, we will give it the highest respect".
"However, "Yes" campaigners argue that such extensive powers are necessary to keep political, economic and security crises at bay, and that concerns over dictatorships and authoritarian regimes are unwarranted.Muttalip Meric, a postgraduate student who voted "Yes", told MEMO that such fears were baseless".
Kurdish militants overnight killed a guard in an attack on a vehicle carrying a district official from Yildirim's ruling AK Party in southeast Turkey's Van province, security sources said.
Many "Yes" voters MEMO spoke to indicated that their vote was not only a vote to empower their presidency, but also "a vote against terrorism", turning their ire towards Daesh militants and leftist extremists from the Kurdistan Workers' Party (PKK), responsible for many bombings in Turkey and mass civilian casualties. "But still they are ahead", he added.
On the eve of the vote, Erdogan held four separate rallies in Istanbul, urging supporters to turn out in large numbers. "Call all your friends, family members, acquaintances, and head to the polls", he said.
Kemal Kilicdaroglu, chief of the Republican People's Party's (CHP), the main opposition party in Turkey, gives a speech on April 12, 2017 in Istanbul during a campain meeting for the "No" side. Under the changes, President Erdogan could stay in power through 2029.
Closely watched on Monday will be the initial assessment of the worldwide observer mission of the OSCE Office for Democratic Institutions and Human Rights (ODIHR) and the Parliamentary Assembly of the Council of Europe (PACE). "If someone, whether Erdogan or any future president, is clearly unfit to rule, even his own party will see that and could vote against him in parliament".