Turkish President Recep Tayyip Erdogan has narrowly won a referendum to expand presidential powers, which could keep him in office until 2029.
With 99.45% of ballots counted, the "Yes" campaign had won 51.37% and "No" 48.63%, and the electoral board called victory for "Yes".
Erdogan supporters say replacing the parliamentary system with an executive presidency will modernise the country.
Turkey's two main opposition parties said they would challenge the results.
The Republican People's Party (CHP) demanded a recount of 60% of votes. They criticised a decision to accept unstamped ballot papers as valid unless proven otherwise.
As jubilant Erdogan supporters rallied in the big cities, pots and pans were banged in Istanbul by opponents of the referendum, in a traditional form of protest.
Three people were shot dead near a polling station in the south-eastern province of Diyarbakir, reportedly during a dispute over how they were voting.
The European Commission called on the Turkish authorities in a statement to "seek the broadest possible national consensus" when implementing the constitutional reforms.
Lingering doubts: BBC's Mark Lowen in Ankara
They are rejoicing into the night here outside the headquarters of the governing AK party (AKP), confident in the victory claimed by President Erdogan.
He and his government say more than 51% of voters have backed the constitutional reform but the opposition has cried foul, claiming massive irregularities over invalid votes and vowing to challenge the result at the supreme electoral board.
Mr Erdogan said the clear victory needed to be respected. In a typically rabble-rousing speech, he proposed another referendum on reinstating the death penalty, which would end Turkey's EU negotiations.
But this has not been the resounding win he wanted and doubts will linger over its legitimacy. It was hoped this vote might bring Turkey stability but that still seems some way off.
Death penalty next?
"Today... Turkey has taken a historic decision," Mr Erdogan told a briefing at his official Istanbul residence, the Huber Palace. "With the people, we have realised the most important reform in our history."
He called on everyone to respect the outcome of the vote.
The president also said the country could hold a referendum on bringing back the death penalty.
He usually gives triumphant balcony speeches, the BBC's Mark Lowen notes, but this was a muted indoors address.
Deputy Prime Minister Veysi Kaynak admitted the "Yes" vote had been lower than expected.
What's in the new constitution?
The draft states that the next presidential and parliamentary elections will be held on 3 November 2019.
The president will have a five-year tenure, for a maximum of two terms.
- The president will be able to directly appoint top public officials, including ministers
- He will also be able to assign one or several vice-presidents
- The job of prime minister, currently held by Binali Yildirim, will be scrapped
- The president will have power to intervene in the judiciary, which Mr Erdogan has accused of being influenced by Fethullah Gulen, the Pennsylvania-based preacher he blames for the failed coup in July
- The president will decide whether or not impose a state of emergency
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 new system, he argues, will resemble those in France and the US and will bring calm in a time of turmoil marked by a Kurdish insurgency, Islamist militancy and conflict in neighbouring Syria, which has led to a huge refugee influx.
Critics of the changes fear the move will make the president's position too powerful, arguing that it amounts to one-man rule, without the checks and balances of other presidential systems such as those in France and the US.
They say his ability to retain ties to a political party - Mr Erdogan could resume leadership of the AKP he co-founded - will end any chance of impartiality.
CHP deputy leader Erdal Aksunger said he believed there had been irregularities in the count: "Many illegal acts are being carried out in favour of the 'Yes' campaign right now.
"There is the state on one side and people on the other. 'No' will win in the end. Everybody will see that."
The pro-Kurdish Peoples' Democratic Party (HDP) also challenged the vote.
Many Turks already fear growing authoritarianism in their country, where tens of thousands of people have been arrested, and at least 100,000 sacked or suspended from their jobs, since a coup attempt last July.
The campaign unfolded under a state of emergency imposed in the wake of the failed coup.
Mr Erdogan assumed the presidency, meant to be a largely ceremonial position, in 2014 after more than a decade as prime minister.
Under his rule, the middle class has ballooned and infrastructure has been modernised, while religious Turks have been empowered.
Relations with the EU, meanwhile, have deteriorated. Mr Erdogan sparred bitterly with European governments who banned rallies by his ministers in their countries during the referendum campaign. He called the bans "Nazi acts".
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