The Organization for Security and Co-operation in Europe election monitor said Turkey's referendum was not impartial and instead favored President Recep Tayyip Erdogan amid "limitations on fundamental freedoms".
Despite a state of emergency and a widespread crackdown on dissent, Erdogan succeeded in persuading only 51.4% of voters to back his constitutional upheaval. The result has exposed divisions in Turkish society; the country's three biggest cities-Istanbul, Ankara and Izmir-all voted against the constitutional changes.
He made the comments at news conference in Ankara.
The margin fell short of the sweeping victory the 63-year-old Erdogan had sought in the referendum. But the narrowness of his victory could have the opposite effect: adding to volatility in a country that has lately survived an attempted coup, attacks by Islamists, a Kurdish insurgency, civil unrest and war across its Syrian border. "His tendency has been not to co-opt the opposition but to crush it".
Critics accuse Erdogan of steering Turkey towards one-man rule.
Erdogan told supporters on Sunday that Turkey could hold another referendum on reinstating the death penalty. "The crusader mentality attacked us overseas, inside their lackeys attacked us".
"We look forward to OSCE/ODIHR's final report, which we understand will take several weeks", acting spokesman Mark Toner said in a statement.
Turkey's main opposition party, the Republican People's Party, has vowed to contest irregularities with Turkey's Constitutional Court and the European Court of Human Rights. The electoral board insists the referendum result is valid, however.
"At the moment it is impossible to determine how many such votes there are and how many were stamped later".
He said it was not possible for authorities to determine how many ballot papers may have been irregularly cast.More news: Trump considering to introduce sanctions against Russia, Iran for supporting Syria
The OSCE cannot sanction Turkey, it can only make recommendations.
Authorities said the decision to accept unstamped ballots was legitimate because it was taken before voting began.
"The ballot papers are not fake, there is no (reason) for doubt", said Guven. However, this referendum is a decision on a new administrative system, a change and a transformation in the Republic of Turkey. "There is a serious and solid problem of legitimacy that will forever be debated".
If the Yes vote prevails, the Turkish prime minister's office is set to be abolished and replaced with a presidential system.
The amendments will also allow the president to appoint vice president, ministers and some members of the Board of Judges and prosecutors.
Erdogan has adopted an increasingly combative tone toward the European Union in recent months. The AP reports that supporters of the "yes" vote have dominated the airwaves, while supporters of the "no" vote have complained of intimidation. The vote, he said, demonstrated that Turkish people had said, "Yes to a single nation".
Ismail Calisan, an Ankara resident, accepted the result with grace.
Merkel and Gabriel, whose country has about 3 million residents of Turkish background, said they noted the preliminary result showing a victory for the "Yes" camp.
The result laid bare the deep divide between the urban middle classes who see their future as part of a European mainstream, and the pious rural poor who favour Erdogan's strong hand.