Just as polls were closing Sunday in Turkey for a referendum on constitutional amendments that would broaden the president's powers, the country's election authority took an unusual step: It lifted a rule requiring that each ballot have an official seal.

The ballot for Turkey's referendum was remarkably simple. A thin piece of paper no longer and wider than a bar of chocolate, it contained only two choices: Evet (Yes) on the white side of the ballot or Hayir (No) on the brown side of the ballot. Voters were given an ink stamp and instructed to mark the side of their choice.

Each official ballot was to be stamped with an official seal before being handed to voters. The seal, as one might expect, is meant to signify that the vote cast was valid.

But Turkey's Supreme Election Board, or YSK, on Sunday, just as the polls were closing, changed the rule requiring each ballot to be stamped with a seal. Instead, questionable ballots with no official seal were to be considered valid unless there was proof that they were fraudulent.

Now, the main opposition parties are claiming that as many as 2.5 million “unsealed” ballots — not bearing the official stamp — were cast and counted. In past elections, those votes would have been invalid.

The Republican People's Party, or CHP, on Monday demanded the annulment of the result. The pro-Kurdish Peoples' Democratic Party, or HDP, has also vowed to contest the results.

Those efforts were buoyed Monday by a report from an international monitoring group — which includes the Organization for Security and Cooperation in Europe — that found overall that the referendum “took place on an unlevel playing field.” The report said the YSK's decision to change the ballot procedures “removed an important safeguard.”

Turkish President Recep Tayyip Erdogan criticized the report, saying, “The crusader mentality in the West and its servants at home have attacked us,” Reuters reported.

After the rule change Sunday, allegations of voter fraud emerged on social media.

In a statement, the head of the YSK said the official results will be released in 11 to 12 days, after the opposition's objections have been considered.

Sunday's claims of voter fraud and the ballot rule change come amid an extended state of emergency in place since a failed coup in July 2016.

In the wake of the attempted coup, the AKP has carried out a systematic purge of dissidents and other enemies within state institutions, schools and universities. Opposition politicians, journalists and others were detained. According to the report by the referendum monitoring group, 100,000 people have been prosecuted, 40,000 detained and more than 150,000 dismissed from civil positions.

The report also cites the Venice Commission, which notes, "The current state of emergency does not provide for the due democratic setting for a constitutional referendum."

Read more about Turkey's referendum: