ISTANBUL >> Turkish President Recep Tayyip Erdogan has finally fulfilled his long-held ambition to expand his powers after Sunday’s referendum handed him the reins of his country’s governance. But success did not come without a cost.

His victory leaves the nation deeply divided and facing increasing tension with former allies abroad, while international monitors and opposition parties have reported numerous voting irregularities.

An unofficial tally carried by the country’s state-run news agency gave Erdogan’s “yes” vote a narrow win, with 51.4 percent approving a series of constitutional changes converting Turkey’s political system from a parliamentary to a presidential one. Critics argue the reforms will hand extensive power to a man with an increasingly autocratic bent, leaving few checks and balances in place.

Opposition parties called foul, complaining of a series of irregularities. They were particularly outraged by an electoral board decision to accept ballots that did not bear official stamps, as required by Turkish law, and called for the vote to be annulled. International monitors from the Organization for Security and Cooperation in Europe, who also listed numerous irregularities, said the move undermined safeguards against fraud.

The referendum campaign was heavily weighted in favor of the “yes” campaign, with Erdogan drawing on the full powers of the state and government to dominate the airwaves and billboards. The “no” campaign complained of intimidation, detentions and beatings.

In Istanbul, hundreds of “no” supporters demonstrated in the streets on Monday, chanting “thief, murderer, Erdogan” and banging pots and pans.

“We are protesting today because the results announced by the government are not the real ones. Because actually the ‘no’ we voted won. But the government is announcing it as ‘yes’ has won,” Damla Atalay, a 35-year-old lawyer, said of the voting irregularities.

Erdogan was unfazed by the criticism as he spoke to flag-waving supporters in the Turkish capital, Ankara.

“We have put up a fight against the powerful nations of the world,” he said as he arrived at the airport from Istanbul. “The crusader mentality attacked us abroad. … We did not succumb. As a nation, we stood strong.”

In a speech before a massive crowd at his sprawling presidential palace complex, Erdogan insisted Turkey’s referendum was “the most democratic election … ever seen in any Western country” and admonished the OSCE monitors to “know your place.”

The increasing polarization of Turkish society has long worried Turkey observers, who note the dangers of deepening societal divisions in a country with a history of political instability.

The referendum was held with a state of emergency still in place, imposed after an attempted coup in July. About 100,000 people have been fired from their jobs in the crackdown that followed on supporters of a U.S.-based Islamic cleric and former Erdogan ally who the president blamed for the attempted putsch. Tens of thousands have been arrested or imprisoned, including lawmakers, judges, journalists and businessmen.

On Monday, the country’s Council of Ministers decided to extend the state of emergency, which grants greater powers of detention and arrest to security forces, for a further three months. It had been due to expire April 19. The decision was to be sent to parliament for approval.

“The way (Erdogan) has closed the door on the opposition, there is likely to be increased political unrest,” said Howard Eissenstat, associate professor of Middle East history at St. Lawrence University in upstate New York. “Forty-eight percent of the population is being told that their voices don’t matter.”

There is also the risk of increased international isolation, with Erdogan appealing to patriotic sentiments by casting himself as a champion of a proud Turkish nation that will not be dictated to by foreign powers in general, and the European Union in particular.

Turkey has been an EU candidate for decades, but its accession efforts have been all but moribund for several years.

“They have made us wait at the gates of the European Union for 54 years,” Erdogan told his supporters at the presidential palace. “We can conduct a vote of confidence on this as well. Would we? What did England do — they did Brexit, right?”

“Either they will hold their promises to Turkey or they’ll have to bear the consequences,” he added.

Erdogan has also vowed to consider reinstating the death penalty — a move that would all but end prospects of EU membership. But, he insisted, other nations’ opinions on the issue are irrelevant to him.

“Our concern is not what George or Hans or Helga says. Our concern is what Hatice, Ayse, Fatma, Ahmet, Mehmet, Hasan, Hüseyin says,” he thundered as the crowd of supporters chanted for the return of capital punishment. “What Allah says. That’s why our parliament will make this decision.”

Both Germany and France expressed concern about possible election irregularities and called on Erdogan to engage in dialogue with the opposition.

“The narrow result of the vote shows how deeply split the Turkish society is,” German Chancellor Angela Merkel and Foreign Minister Sigmar Gabriel said in a joint statement. “This implies a big responsibility for the Turkish government and President Erdogan personally.”

The U.S. State Department “encouraged voters and parties on both sides to focus on working together for Turkey’s future,” while calling on the government to protect rights and freedoms “regardless of their vote on April 16.”

The words are unlikely to move Erdogan.

Relations with Germany and the Netherlands have been particularly tense, with Erdogan outraged by decisions in both countries not to allow his government to campaign there to woo the expatriate vote for the referendum.

The referendum approves 18 constitutional amendments to replace Turkey’s parliamentary system with a presidential one.

The president will be able to appoint ministers, senior government officials and to hold sway over who sits in Turkey’s highest judicial body, as well as to issue decrees and declare states of emergency. They set a limit of two five-year terms for presidents.

The new system takes effect at the next election, currently slated for 2019. Other changes are to be implemented sooner, including scrapping a requirement that the president not be a member of any political party. This would allow Erdogan to rejoin the governing AK Party he co-founded, or to lead it.

“Erdogan dominated the national media. He imposed a very restrictive environment for the ‘no’ camp,” said Fadi Hakura, a Turkey specialist at the London-based Chatham House think tank. “He secured a thin majority of 1 percent. This suggests that Erdogan will become more robust and more challenging to deal with.”