U.S. President Donald Trump has congratulated Turkish President Recep Tayyip Erdogan on his “referendum victory,” in a narrow vote that would create a powerful executive presidency from the current parliamentary system.

The White House said in a statement the two leaders spoke by phone, with their conversation also including the need to hold Syrian President Bashar al-Assad accountable for a recent chemical attack, the ongoing fight against Islamic State and “the need to cooperate against all groups that use terrorism to achieve their ends.”

Erdogan’s opponents are seeking a revote of Sunday’s referendum, and international monitors have questioned the fairness of the vote, saying it was contested on an uneven playing field.

At a news conference Monday in Ankara, monitors from the Organization for Security and Cooperation in Europe said the “No” campaign faced numerous obstacles including a lack of freedom of expression, intimidation and access to the media. The OSCE also alleged misuse of administrative resources by Erdogan.

 

The controversial decision to allow the use of ballots that did not have an official stamp was also criticized. “The Supreme Election Board issued instructions late in the day, that significantly changed, the validity criteria, undermining an important safeguard and contradicting the law,“ observed Cezar Florin Preda of the monitoring group at the Ankara press conference

 

Turkey’s Foreign Ministry released a statement saying it was “saddened” by the OSCE’s finding that the referendum fell short of international standards. The ministry called it “unacceptable” and accused the OSCE of political bias.

 

Under Turkey’s 2010 electoral law, all ballots require an official stamp as a measure aimed at preventing vote stuffing. The main opposition CHP alleges that as many as one-and-a-half million unstamped ballots could have been used, more than the winning margin in the referendum.

The CHP is now demanding the referendum be held again. “The only decision that will end debate about the legitimacy, and ease the people’s legal concerns is the annulment of this election,” declared Bulent Tezcan CHP deputy head, speaking at a press conference Monday.

Prime Minister Binali Yildirim rejected opposition complaints in remarks to a group of legislators Tuesday. He said the opposition “should not speak after the people have spoken.”

Protests were held in several locations across Istanbul and in the capital, Ankara, over the handling of the vote; similar demonstrations were reported in other cities.

The only legal redress the CHP has to overturn the vote is with Supreme Election Board, which made the decision to use the unstamped ballots.

 

The head of the board, Sadi Guven, strongly defended his decision to allow the controversial ballots, citing high demand for ballots and saying similar procedures had been followed in the past.

“This is not some move we’ve done for the first time,” said Guven, speaking to reporters Monday in Ankara. “Before our administration took over, there had been many decisions approving the validity of unstamped ballots.”

 

Critics point out the previous use of unstamped ballots was before the introduction of the electoral law banning the practice. Guven said he did not know how many of the ballots were used, and admitted he made the decision after consulting with the ruling AK Party.

 

Many of the ballots are suspected of being used in the predominantly Kurdish southeast where strict security measures are in force due to an ongoing fight against Kurdish insurgent group the PKK. “No” campaigners in the region, said its observers, were prevented from monitoring many ballot stations. The OSCE also said its monitors too faced restrictions.

 

While the OSCE refused to be drawn in on whether the shortcomings and difficulties it highlighted were enough to ultimately affect the outcome of the vote, its assessment will likely embolden the opposition and add to growing international concern.

“The European politician will refer to the OSCE; even Americans have said it was going to wait for the OSCE report [before commenting on the referendum result], warns political columnist Semih Idiz of Al Monitor website. “It’s a complication for Erdogan but he will try and turn it to his advantage, by saying the West is up to its old tricks again.” Throughout the campaign, Erdogan played the nationalist card, accusing Western countries of conspiring against him and Turkey. Erdogan described the referendum as a victory against the crusaders.

Europe has so far avoided directly addressing the controversy, choosing to look beyond the result with calls on Erdogan to reach out to his opponents to ease the political polarization. The U.S. State Department called on Turkey to protect basic rights and freedoms as authorities work to resolve the contested results.