ISTANBUL — A vote to grant new powers to President Recep Tayyip Erdogan sparked fresh arguments in a divided Turkey on Monday, as opposition parties called for the annulment of the referendum results and Erdogan insisted the debate over the outcome should stop.

And a sharply worded report Monday by an international monitoring group said the referendum “fell short” of full adherence to international standards. It criticized numerous aspects of the vote, including a change to the ballot-counting procedures that “removed an important safeguard.”

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 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 Islamic State in neighboring Syria.

Hopes that the end of the rancorous referendum campaign might suppress some of the country’s feuds seemed to evaporate late Sunday night, as Erdogan’s supporters flooded streets in flag-waving celebrations, his opponents marched with clanging pots in protest and political parties yelled foul at the result.

In a combative speech at the presidential palace in Ankara on Monday evening, Erdogan dismissed the protesters as well as the opposition party complaints, saying that the “debate about this issue is now over.”

And he warned the European observers who had criticized the election 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 a crowd cheered. A few hours away, in rainy Istanbul, for a second night, the pot-clanging protesters prepared to take to the streets.

The 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.

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 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 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.”