Turkish President Recep Tayyip Erdogan arrives to deliver a speech at the headquarters of his ruling Justice and Development Party, or AKP, in Istanbul on Sunday. (Bulent Kilic/AFP/Getty Images)

President Recep Tayyip Erdogan on Monday vigorously defended his narrow victory in a referendum granting him greater authority, dismissing criticism of the result by Turkish opposition parties, European election observers and protesters while making clear his government was moving on.

“Debate about this issue is now over,” he said in an occasionally combative speech at his sprawling presidential palace that mentioned the 25 million voters who had supported the measure. “We are not going to stop,” he said, according to an English translation of his comments on a state news channel.

His comments came hours after Turkey’s main opposition party demanded that the vote be annulled because of irregularities, and after the European observers released a preliminary report saying the vote “fell short” of adherence to international standards. The complaints, along with scattered protests, dashed hopes that Turkey’s rancorous debates over the referendum would subside after the vote. 

By a razor-thin margin, Turkish voters on Sunday approved constitutional changes that will radically transform the country’s system of government, abolishing the post of prime minister and shifting from a parliamentary system. The new model strengthens the clout of the presidency just eight months after a coup attempt aimed at toppling Erdogan’s government.

The outcome has laid bare deep political divisions in Turkey and could have wider resonance in everything from Turkey’s decades-old bid for membership in the European Union and Turkish interactions within NATO to the U.S.-led coalition fighting the Islamic State in neighboring Syria.

A statement from Erdogan’s office said President Trump spoke with the Turkish leader Monday night and congratulated him on the referendum win. The two also discussed the war in Syria, the statement added.

A primary opposition complaint is that, as voting was underway, Turkey’s election board decided to accept ballots that were not stamped with an official seal, in contravention of the law. Election board officials have said they were trying to avoid suppressing votes and that the decision was not unprecedented in Turkey’s elections. 

Bulent Tezcan, deputy leader of the Republican People’s Party, or CHP, Turkey’s main opposition party, said Monday that “only one decision” could calm the nation — “for the referendum to be canceled by the Supreme Election Board.”

By accepting unstamped ballots, the election board “changed the rules of the game halfway through the match,” he said.  

The criticism by the European observers went far beyond the voting irregularities, though, and was more broadly critical of the government’s conduct in the run-up to the referendum. The preliminary report by the observers, from a joint mission by the Council of Europe and the Organization for Security and Cooperation in Europe, said the referendum took place on an “unlevel playing field and the two sides of the campaign did not have equal opportunities.”  

Erdogan’s opponents had been making the same charge for months. A government crackdown after the failed coup attempt in July had gone far beyond the coup plotters, leading to arrests of prominent opposition politicians and prosecutions of journalists critical of the government.  

As a result, the election observers said, “one side’s dominance in the coverage and restrictions on the media reduced voters’ access to a plurality of views.”  

“Under the state of emergency put in place after the July 2016 failed coup attempt, fundamental freedoms essential to a genuinely democratic process were curtailed,” the observers added.

A Turkish Foreign Ministry statement accused the observers of having “disregarded the principles of objectivity,” calling the report a reflection of a “biased” approach. In his speech at the palace, Erdogan warned the European observers to “know your limits” as they prepared their final report, which he also dismissed, in advance.  

“We will not see it, we will not hear it, we will not accept it,” he said as his supporters cheered.

In Europe, leaders noted the serious rifts in Turkey and urged Erdogan not to risk possible new blows to the already strained relations between the European Union and Turkey. 

Before the referendum, Erdogan had leveled sharp criticism against Europe — including labeling some leaders as Nazis —for halting political events among the millions of Turks living on the continent.

German Chancellor Angela Merkel said the “tight outcome of the referendum shows how deeply split Turkish society is” and appealed for Turkish leaders to seek “respectful dialogue” with opposition groups. “Erdogan personally needs to take on a great responsibility,” she said.

State Department acting spokesman Mark Toner also acknowledged the claims by Turkey’s opposition and appealed to the Turkish government to protect basic rights.

Financial markets, however, appeared to shrug off the discord. The Turkish lira and the country’s main stock exchange rose.

The gravity of the issues in the referendum swelled turnout on Sunday, bringing out 85 percent of voters, including people who called themselves apolitical or said they were unaffiliated with any of Turkey’s major parties.

Changes to the constitution would allow Erdogan, 63, who served as prime minister for 11 years before becoming president, to run for reelection in 2019 and serve two terms that would end in 2029.  

Ebru Alacam, a 40-year-old insurance agent in Istanbul, said Sunday that she had voted no on the changes, adding that she was not a “political person.”  

But the referendum seemed like a turning point for Turkey, she said. “I don’t want a single leader. I don’t want a single party. I want pluralism.” It was not her intent to disregard the views of Erdogan’s supporters, she said, but added that their preferences could not be paramount.  

“This country has many identities,” Alacam said.

But Ahmed Colak, a 54-year-old stock investor, had cheered Erdogan’s thunderous attacks on the Europeans during the referendum campaign, saying “it had stirred nationalism in me.”  

He was not a member of the ruling Islamist Justice and Development Party but valued Erdogan’s leadership, citing large-scale development projects initiated by Erdogan, and smaller accomplishments such as the garbage in his neighborhood that, during the president’s tenure, has disappeared from the streets.  

Brian Murphy in Washington contributed to this report.