Turkey's main opposition party is demanding a referendum granting President Tayyip Erdogan sweeping new powers be nullified after a narrow "Yes" vote that exposed bitter divisions and drew concern from European Union leaders.
Mr Erdogan's supporters took to the streets to cheer, while opponents stayed indoors banging pots and pans in protest over the vote to bring the biggest overhaul in Turkish politics since the founding of the modern republic, abolishing the prime minister's post and concentrating power in the presidency.
Unofficial results showed a narrow victory with 51.4 percent of votes cast in favour. Official results are due in 12 days.
Mr Erdogan, a populist with a background in once-banned Islamist parties, has ruled since 2003 with no real rival, while his country emerged as one of the fastest-growing industrial powers in both Europe and the Middle East.
He has also been at the centre of global affairs, commanding NATO's second-biggest military on the border of Middle East war zones, taking in millions of Syrian refugees and controlling their further flow into Europe.
Critics accuse him of steering Turkey towards one-man rule and exacerbating divisions that could increase instability. The two largest opposition parties both challenged Sunday's referendum, saying it was deeply flawed.
The pro-Kurdish opposition Peoples' Democratic Party said it presented complaints about unstamped ballots affecting three million voters, more than twice the margin of Erdogan's victory.
The main secularist opposition People's Republican Party said it was still unclear how many votes were affected.
"This is why the only decision that will end debate about the legitimacy (of the vote) and ease the people's legal concerns is the annulment of this election," deputy party chairman Bulent Tezcan said.
Mr Tezcan said he would if necessary go to Turkey's constitutional court - one of the institutions that Erdogan would gain firm control over under the constitutional changes, through the appointment of its members.
Mr Erdogan has long said the changes were needed to end chronic instability that plagued the country over decades when the military repeatedly tried to seize power from weak civilian governments.
"For the first time in the history of the Republic, we are changing our ruling system through civil politics," he said in a victory speech.
But the divisions revealed by the narrow referendum result could also herald more unrest to come. The changes won strong backing in conservative rural areas, but were just as strongly opposed in Istanbul and other cities, as well as in the restive Kurdish southeast.
The president survived a coup attempt last year and responded with a crackdown, jailing 47,000 people and sacking or suspending more than 120,000 from government jobs such as schoolteachers, soldiers, police, judges or other professionals.
The changes could keep him in power until 2029 or beyond, making him easily the most important figure in Turkish history since state founder Mustafa Kemal Ataturk built a modern nation from the ashes of the Ottoman empire after World War I.
Response from Europe, which has had increasingly strained ties with Turkey, was cautious, awaiting the judgment of international observers later on Monday. Germany, host to some four million Turks, said it was up to Erdogan himself to heal the rifts that the vote had exposed.
"The tight referendum result shows how deeply divided Turkish society is, and that means a big responsibility for the Turkish leadership and for President Erdogan personally," said Chancellor Angela Merkel and Foreign Minister Sigmar Gabriel in a joint statement.
Mr Erdogan said 25 million people had supported the proposal, which will replace Turkey's parliamentary system with an all-powerful presidency. That was a smaller mandate than the decisive result for which he and his ruling AK Party had aggressively campaigned.
Under the changes, most of which will only come into effect after the next elections due in 2019, the president will appoint the cabinet and an undefined number of vice-presidents, and be able to select and remove senior civil servants without parliamentary approval.