Votes for constitutional change to hand President Tayyip Erdogan sweeping powers held a narrow lead with nearly all ballot boxes opened on Sunday, but Turkey's three largest cities and the mainly Kurdish southeast looked set to vote "No".
The vote in the southeastern Turkish city of Diyarbakir started one hour earlier at 7am (0400 GMT).
Although voting in elections and referendums is compulsory in Turkey, Sunday's turnout of more than 85 per cent of the 55 million Turks eligible to vote was considered high.
I watched as queues of voters turned up to a polling station in Ankara where the voting process was quick and efficient.
Aksunger said: "Since this morning, we have determined some 2.5 million problematic votes". His wife Zeynep agreed, saying: "I was going to come sleep here last night to vote at first light".
The "Yes" share of the vote - which stood at 63 percent after around one quarter had been opened - eased as the count moved further west towards Istanbul and the Aegean coast. "Yes, yes, yes. Our leader is the gift of God to us".
Supporters found in him a man who gave a voice to the working- and middle-class religious Turks who had long felt marginalized by the country's Western-leaning elite. "He's governing so well", Mualla Sengul said.
Erdogan has also blasted European countries, accusing the Netherlands of Germany of being Nazis after authorities there refused to allow Turkish ministers to hold rallies to woo expatriate voters for the referendum.
According to the BBC. under the new amendments, Erdogan would be given enhanced powers to appoint cabinet ministers, issue decrees, choose senior judges and dissolve parliament.
The constitutional changes will only enter into force after the presidential and parliamentary elections set for 2019.
A "Yes" vote would replace Turkey's parliamentary democracy with an all-powerful presidency and may see Erdogan in office until at least 2029, in the most radical change to the country's political system in its modern history.
The referendum campaign was divisive and heavily one-sided, with the "yes" side dominating the airwaves and billboards across the country. Supporters of the "no" vote have complained of an atmosphere of intimidation, with the main opposition party recording more than 100 incidents of obstruction to its campaign efforts, including beatings, detentions and threats.
As NPR's Peter Kenyon reported in a preview, "the vote comes at a perilous time".
"Mr Erdogan says the changes are needed to address Turkey" s security challenges nine months after an attempted coup, and to avoid the fragile coalition governments of the past.
The campaign unfolded under a state of emergency imposed in the wake of the failed coup. Media outlets are stifled, hundreds of non-governmental organizations and news outlets have been shut down, as have many businesses, from schools to fertility clinics.
The country's foreign minister Mevlut Cavusoglu said this was the birth of a "truly new Turkey".
This once stable corner of the region has in recent years been convulsed by terror attacks and millions of refugees, mostly from Syria, have arrived.