Electoral board says Turkey has backed constitutional amendments but opposition parties say they will contest some of the results

The Turkish president, Recep Tayyip Erdoğan, has achieved victory in a historic referendum on a package of constitutional amendments that will grant him sweeping new powers. Sadi Güven, the head of Turkey’s high electoral board (YSK), confirmed the passage of the referendum on Sunday night, based on unofficial results.The yes campaign won 1.25m more votes than the no campaign, with only about 600,000 votes still to be counted, Güven told reporters in Ankara, meaning the expanded presidential powers had been approved.

However, disparities persisted into Sunday evening, with the opposition saying not all ballots had been counted and they would contest a third of the votes that had been cast. Güven said the YSK had decided to consider unstamped ballots as valid unless they were proved to be fraudulent, after a high number of complaints – including one from the ruling Justice and Development party (AKP) – that its officials had failed to stamp some ballot papers.

The no campaign said the YSK’s last-minute decision raised questions about the validity of the vote. But Güven said the decision was taken before results were entered into the system and that members of the AKP and the main opposition were present at almost all polling stations and signed off on reports. He said official results were expected in 11-12 days.

The result of the referendum sets the stage for a transformation of the upper echelons of the state and changing the country from a parliamentary democracy to a presidential republic, arguably the most important development in the country’s history since it was founded on the ashes of the Ottoman Republic.

Erdoğan said he would immediately discuss reinstating the death penalty in talks with the prime minister and the nationalist opposition leader, Devlet Bahçeli. The president said he would take the issue to referendum if necessary.

The narrow victory will nevertheless come as a disappointment for the country’s leadership, which had hoped for a decisive mandate for the plan that could see Erdoğan remain in power until 2029 if he wins successive elections. The result will set the stage for a further split between Turkey and its European allies, who believe Ankara is sliding towards autocracy. The European commission said on Sunday night that Turkey should seek the “broadest possible national consensus” in its constitutional amendments, given the yes campaign’s slim margin of victory.

Results carried by the state-run Anadolu news agency showed the yes vote had about 51.3% compared with 48.7% for the no vote, with nearly 99% of the vote counted. Turnout exceeded 80%. The country’s three largest cities – Istanbul, Ankara and Izmir – voted against the changes, and so did the vast majority of Kurdish voters and many of the coastal cities, indicating a general decline in the ruling party’s support.

In a press conference in Istanbul following his party’s declaration of victory, Erdoğan said that unofficial results showed there were about 25m yes votes, 1.3m more than no. But in an unusually muted victory speech, Erdoğan said foreign powers should respect the referendum’s outcome. He said: “We’ve got a lot to do, we are on this path but it’s time to change gears and go faster … We are carrying out the most important reform in the history of our nation.”

Erdoğan claimed support for constitutional change had risen in south-east Turkey and hailed a “profound” jump in support for a presidential system that was unpopular just two years ago. Overseas votes were a “big part” of that success, he said, adding that his new executive presidency would probably come into effect after the 2019 election.

Erdoğan called the prime minister, Binali Yıldırım, and other political allies to congratulate them on the victory, although, in an indication of the ruling AKP’s disappointment, the deputy prime minister said they had received fewer votes than they expected.

Yıldırım delivered a victory speech from the balcony at party headquarters in Ankara, where thousands of the party faithful gathered to celebrate the victory. He said: “Our nation has made its decision and said yes to the presidential system. The ballot box result showed we will not bow to traitors and terrorists. Turkey has won; our nation has won.”

He said the government would use this opportunity to make a “brighter Turkey”, adding: “We are brothers. We are one body, one nation. A new page has opened in our democratic history with this vote. Be sure that we will use this result for our people’s welfare and peace in the best way.”