Turkey's main opposition party on Monday urged the country's electoral board to cancel the results of a landmark referendum that granted sweeping new powers to the nation's president, citing what it called substantial voting irregularities.

International observers who monitored the voting also found irregularities, saying the conduct of Sunday's referendum "fell short" of international standards. It specifically criticized a decision Sunday by Turkey's electoral board to accept ballots that did not have official stamps, saying that undermined safeguards against fraud.

Turkey's electoral board confirmed the Yes victory in the referendum and said the final results would be declared in 11 to 12 days. The state-run Anadolu Agency said the Yes side stood at 51.4 per cent of the vote, while the No vote saw 48.6 per cent support.

The margin could cement President Recep Tayyip Erdogan's hold on power in Turkey for a decade and is expected to have a huge effect on the country's long-term political future and its international relations. Opponents had argued the constitutional changes give too much power to a man they say has shown increasingly autocratic tendencies.

A member of an electoral committee holds a ballot, with the words, which shows the options of Yes or No, during a counting procedure inside a polling station in Ankara, Turkey, on Sunday. (Burhan Ozbilici/Associated Press)

On Monday, Erdogan slammed his critics at home and abroad.

"We have put up a fight against the powerful nations of the world," he told supporters greeting him at Ankara airport, after arriving from Istanbul. "The crusader mentality attacked us abroad, inside their lackeys attacked us. We did not succumb; as a nation we stood strong."

Opposition parties, however, cried foul on the vote. Bulent Tezcan, deputy chairman of the Republican People's Party, or CHP, cited numerous problems in the conduct of the vote.

"This is not a text of social consensus but one of social division," Tezcan said. "There is a serious and solid problem of legitimacy that will forever be debated."

U.S. President Donald Trump in a phone call congratulated Erdogan on the referendum results, according to a White House statement released Monday.

Trump also thanked Erdogan for supporting the recent U.S. strikes in Syria, and the two discussed combating ISIS, the statement said.

Unstamped ballots accepted

An unprecedented electoral board decision to accept as valid ballots that didn't bear the official stamp has led to outrage.

Turkish president Recep Tayyip Erdogan declared victory in a historic referendum that would grant him sweeping new powers. The opposition says it's a recipe for dictatorship and plans to challenge it. 2:36

Normally, for a ballot to be considered valid, it must bear the official stamp on the back, be put into an envelope that also bears an official stamp and be handed to the voter by an electoral official at a polling station. The system is designed to ensure that only one vote is cast per registered person and to avoid the possibility of ballot box-stuffing.

The board announced, however, that it would accept unstamped envelopes as valid after many voters complained about being handed blank envelopes that did not bear the official stamp. The board said the ballot papers would be considered invalid only if it was proven they were fraudulently cast.

"There is only one way to end the discussions about the vote's legitimacy and to put the people at ease, and that is for the Supreme Electoral Board to cancel the vote," Tezcan said.

He said it was not possible for authorities to determine how many ballot papers may have been irregularly cast.

Supporters of No vote chant slogans as they protest in Istanbul on Monday. (Cansu Alkaya/Associated Press)

Tana de Zulueta, head of the observer mission of the Organization for Security and Co-operation in Europe, said the ballot decision undermined important safeguards against fraud and contradicted Turkey's own laws.

The monitoring group described a series of irregularities in the referendum, including a skewed pre-vote campaign in favour of the Yes vote, the intimidation of the No campaign and the fact that the referendum question was not listed on the ballot.

De Zulueta said, overall, the procedures "fell short of full adherence" to the standards Turkey had signed up for. The OSCE cannot sanction Turkey for its conduct of the vote but it can suggest recommendations.

Electoral board head Sadi Guven rejected opposition claims of foul play, saying none of the ballot papers declared valid were "fake" or fraudulently cast. Guven said the decision was made so that voters who were by mistake given unstamped ballot papers would not be "victimized."

"The ballot papers are not fake, there is no [reason] for doubt," Guven said.

The referendum approves 18 constitutional amendments that will replace Turkey's parliamentary system of governance with a presidential one.

'Dubious vote'

The changes allow the president to appoint ministers, senior government officials and half the members of Turkey's highest judicial body, as well as to issue decrees, set a limit of two five-year terms for presidents and declare states of emergency — Turkey's Council of Ministers on Monday decided to extend for a further three months a state of emergency declared in the wake of a failed July 2016 coup, though the decision still needs parliamentary approval.

The new presidential system takes effect at the next election, currently slated for 2019. Other changes will take effect sooner, including scrapping a clause requiring the president to be impartial, allowing Erdogan to regain membership of the ruling party he founded — or even to lead it.

A man sells flags with Turkey's founder Mustafa Kemal Ataturk near the Eminonou mosque in Istanbul on Monday. (Petros Karadjias/Associated Press)

CHP legislator Utku Cakirozer told The Associated Press his party would file official objections Monday to results at local electoral board branches, before taking their case to the Supreme Electoral Board.

"At the moment, this is a dubious vote," he said.

The country's pro-Kurdish party said it may take the case to the European Court of Human Rights if the electoral board does not reverse its decision and nullify the ballots lacking official stamps.​