Votes for constitutional change to hand President Tayyip Erdogan sweeping powers held a narrow lead with almost all ballot boxes opened, but Turkey's three largest cities and the mainly Kurdish southeast looked set to vote "No".
The "Yes" votes stood at 51.7 per cent after 95 per cent of ballots had been opened, state-run Anadolu news agency said, with the lead narrowing in the final stages of an increasingly tight count.
A "Yes" vote would replace Turkey's parliamentary democracy with an all-powerful presidency and may see Erdogan in office until at least 2029.
The outcome will also shape Turkey's strained relations with the European Union. The NATO member state has curbed the flow of refugees from wars in Syria and Iraq into the bloc but Erdogan says he may review the deal after the vote.
In Turkey's three biggest cities - Istanbul, Izmir and the capital Ankara - the "No" camp appeared set to prevail narrowly, according to Turkish television stations.
Speaking to reporters in Ankara, Deputy Prime Minister Veysi Kaynak said the "Yes" camp had not won as many votes as expected.
Voting at a school near his home in Istanbul, Erdogan said: "God willing I believe our people will decide to open the path to much more rapid development."
The "Yes" percentage of the vote - which stood at 63 per cent after around one quarter had been opened - eased as the count came further west towards Istanbul and the Aegean coast.
The opposition People's Republican Party (CHP) said a last-minute decision by the electoral board to accept unstamped ballots as valid votes put the vote in question.
"We will pursue a legal battle. If the irregularities are not fixed, there will be a serious legitimacy discussion," CHP deputy chairman Bulent Tezcan said.
Erdogan and his supporters say the changes are needed to amend the current constitution, written by generals following a 1980 military coup.
The government says Turkey, faced with conflict to the south in Syria and Iraq, and a security threat from Islamic State and PKK militants, needs strong leadership to combat terrorism.
Opponents say it is a step towards greater authoritarianism in a country where some 47,000 people have been jailed pending trial and 120,000 sacked or suspended from their jobs in a crackdown following a failed coup last July.
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.
In the southeastern Diyarbakir province, two people were killed and one wounded in a gunfight in a village schoolyard which was being used as a polling station, other security sources said. The cause of the clash was not immediately clear.
Erdogan and the AK Party enjoyed a disproportionate share of media coverage in the buildup to the vote, overshadowing the secular main opposition CHP and the pro-Kurdish Peoples' Democratic Party (HDP).
CHP leader Kemal Kilicdaroglu has accused Erdogan of seeking a "one-man regime", and said the proposed changes would put the country in danger.
The package of 18 amendments would abolish the office of prime minister and give the president the authority to draft the budget, declare a state of emergency and issue decrees overseeing ministries without parliamentary approval.
