ISTANBUL — Turkey’s main opposition party on Monday demanded the annulment of a vote that delivered sweeping new powers to President Recep Tayyip Erdogan, saying widespread irregularities during the balloting had undermined its legitimacy.
By a razor-thin margin, voters Sunday approved constitutional changes that will radically transform Turkey’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 tried to topple Erdogan’s government.
The outcome not only underscores the deep political divisions in Turkey, but could have wider resonance in everything from Turkey’s decades-old bid for membership in the European Union and Turkish interactions within NATO and the U.S.-led coalition fighting the Islamic State in neighboring Syria.
Opposition parties quickly challenged the results over a decision by the election board to lift rules requiring that ballots carry an official seal.
Erdogan and his supporters in the Islamist Justice and Development Party called the vote peaceful and fair, and dismissed claims about possible voting flaws as a distraction.
But the escalating opposition calls by opposition parties questioning the vote highlighted the depth of anger among voters who rejected the referendum, including many who feared that the changes would condemn Turkey to autocratic, one-man rule.
Bulent Tezcan, the deputy leader of the Republican People’s Party, or CHP, Turkey’s main opposition party, said on Monday that “there is only one decision to ease the people, and that is to annul this referendum.”
With its decision to accept unstamped ballots, the election board “changed the rules of the game halfway through the match,” he said.
In Europe, leaders noted the serious rifts in Turkey and urged Erdogan not to stray from Western-style political values or 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 countries 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.
French President François Hollande said that his country “takes note” of accusations of voting irregularities, and warned Turkey that moves to depart from European standards — such as Turkey reimposing the death penalty — could “rupture” relations with the European Union.
A sharply worded report Monday by an international monitoring group said that the referendum “fell short” of full adherence to international standards, criticizing numerous aspects of the vote, including a change to the ballot counting procedures that “removed an important safeguard.”
“One side’s dominance in the coverage and restrictions on the media reduced voters’ access to a plurality of views,” said the report from the monitoring group, which included the Organization for Security and Cooperation in Europe.
“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,” it said.
The changes put to the people Sunday would allow Erdogan, who came to power as prime minister in 2003, to run for reelection in 2019 and serve two five-year terms — cementing, in the minds of many here, his status as the most consequential leader since Mustafa Kemal Ataturk, the founder of the Turkish republic.
Ongoing conflicts in the Middle East have made Erdogan a pivotal partner of Western nations in recent years. That is especially true of the United States, which is leading a military coalition to defeat the Islamic State militant group across Turkey’s borders in Iraq and Syria. Turkey also hosts more than 3 million refugees from Syria and has struck a deal with European nations to prevent the refugees from traveling to Europe.
Domestic turmoil, though, has made Turkey an unpredictable ally. A failed coup last summer killed more than 250 people and set off a feverish government purge of its enemies in state institutions — as well as a hunt by Turkey’s government for alleged coup participants who had fled abroad, including to Europe.
Turkey has accused Fethullah Gulen, a Muslim cleric living in exile in the United States, of directing the coup through his vast network of supporters in Turkey. Erdogan’s government has repeatedly requested Gulen’s extradition from the United States, causing friction in the relationship.
Turkey’s rekindled war with Kurdish militants has also stoked tensions with the United States. Erdogan has objected to American support for Syrian Kurdish forces that Turkey regards as an extension of the Kurdistan Workers’ Party, or PKK, which is classified as a terrorist group by Ankara and Washington.
Erdogan’s relatively measured appeal for unity Sunday was in sharp contrast to the tenor of a referendum campaign that saw the government associating its political opponents with terrorists and that further raised tensions with Turkey’s foreign allies.
During the campaign, Erdogan pursued a fight with several European allies, including Germany and the Netherlands. The cause, ostensibly, was a ban on Turkish officials campaigning for votes among Turkish expatriates in Europe. Erdogan used the squabble to maximum effect, deriding German and Dutch leaders as “Nazis” in a series of broadsides that whipped up nationalist support at home.
Terrorist attacks directed at Turkey by Kurdish militants and the Islamic State have kept the country under a state of emergency since the coup. That, along with a crackdown on critical journalists, opposition politicians and other civil society figures, raised questions about whether the referendum could be held under such circumstances.
A leading opposition politician from the pro-Kurdish Peoples Democratic Party, or HDP, was arrested in November, denying the “No” campaign one of its most visible and charismatic voices. As independent media outlets were shuttered or their executives dragged into court, the AKP’s campaign for “Evet,” or “Yes,” dominated Turkey’s airwaves and its public spaces.
“The playing field was never level,” said Semih Idiz, a Turkish political analyst and columnist who writes for the al-Monitor news site. “All the state’s infrastructure and funds went to promote the yes vote,” he said.
The claims of irregularities by opposition parties, along with the split vote, meant “we are going to have a tense period of domestic debate, unless there is some soul-searching on the part of the government and the presidency,” he said.
Erdogan’s most loyal supporters insisted that the referendum vote went beyond the details of the changes and amounted to a verdict on the president’s record. His coalition included his core constituency of conservative Muslims who have delivered unflinching support to Erdogan over the years, regarding him as a champion of their concerns. He also successfully courted a segment of a Turkish nationalist party whose members helped propel the constitutional amendments through the parliament.
“No” voters cited fear of growing autocracy and the government’s clampdown since the failed coup. In the Kurdish city of Diyarbakir, a focal point of the state’s war with the PKK that is now heavily patrolled by Turkish security forces, Veysi Adenli, 46, said he viewed his decision to reject the referendum as a vote for “peace.”
Brian Murphy in Washington contributed to this report.