It is possible that 2.5 million Turkish votes were “manipulated,” changing the results of the recent constitutional referendum, an international observer monitoring the Turkish referendum told ORF radio.

Alev Korun, an Austrian MP and member of the Council of Europe observer mission, said that there is “a suspicion that up to 2.5 million votes could have been manipulated,” as cited by Reuters.

Korun also referred to the Turkish electoral authorities’ move to accept as valid around 1.5 million unstamped ballots and envelopes.

“Actually, the law only allows official voting envelopes. The highest election authority decided however, as it were against the law, that envelopes without official stamp should be admitted.”

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The Austrian official was not the first to criticize the step. The Organization for Security and Co-operation in Europe (OSCE), which oversaw the referendum, also slammed the decision, stating that it had “significantly changed the ballot validity criteria, undermining an important safeguard and contradicting the law.”

On Monday, the Turkish president responded to the OSCE, saying that he “did not see, hear or acknowledge” their reports, while telling the international observers to “know their place.”

The main Turkish opposition party announced that they would present their appeal of the vote later on Tuesday, a day after they called for the outcome to be nullified, citing the use of unstamped ballots as well.

The “only one way to end the discussions about the vote’s legitimacy and to put the people at ease… is for the Supreme Electoral Board to cancel the vote,” deputy chairman of the main opposition Republican Peoples’ Party (CHP), Bulent Tezcan, said on Monday.

He added that it is unlawful to even count the unstamped ballots, and that his party had received complaints from several regions, where people said that they could not vote in privacy.

READ MORE: Erdogan-2029? 'Yes’ camp claims narrow victory in Turkey's referendum to expand president’s powers

The Turkish election authorities lashed out at the accusations, saying that the last-minute decision to count the unstamped ballots was not unprecedented, and blamed the “mistakes made by the members of some election commissions.”

“Due to those mistakes, we have made this decision, not to violate the voters’ rights for the expression of their will,” the chairman of the High Electoral Board (YSK), Sadi Guven, was quoted by HaberTurk as saying.

We won't consider OSCE report on referendum: Erdogan https://t.co/VonrdXi5vwpic.twitter.com/St4m7Ocvoc — ANADOLU AGENCY (ENG) (@anadoluagency) April 17, 2017

On Tuesday, Prime Minister Binali Yildirim also called on the main opposition party to respect the results of the referendum.

“The people’s will has been reflected at the ballot box, and the debate is over,” Yildirim said, as cited by Reuters. “Everyone should respect the outcome, especially the main opposition.”

The Turkish president claimed a tight victory in the referendum set to grant the country’s leader more powers. Erdogan’s ‘yes’ campaign took around 51 percent of the vote.

Following the announcement, thousands of demonstrators took to the streets of Istanbul to protest, carrying banners like “No, we win” and “No has won.” At least a dozen arrests were reported in the Turkish resort city of Antalya.