The leader of Republican People's Party (CHP), Kemal Kilicdaroglu, slammed on Tuesday the decision of Supreme Election Board to count unsealed ballots in the referendum.
However, with academics pinpointing apparent irregularities in the declared voting patterns, Turkey's bar association stepped into the row, saying the last-minute decision by Turkey's electoral board YSK to allow unstamped ballots in the referendum was clearly against the law.
The U.S. State Department said it had taken note of the European monitors' concerns and looked forward to a final report, urging the Turkish government to protect the rights and freedoms of all citizens, however they voted.
A constitutional reform package seeking to introduce an executive presidency in Turkey was approved by 51.4 percent of the voters in Sunday's referendum. But the global electoral monitor said that while the referendum's technical processes were well administered, there were key issues related to media coverage and the circumstances under which the vote occurred, such as Turkey's ongoing state of emergency enacted after a failed coup d'etat attempt previous year.
Global election monitors, including the Organisation for Security and Cooperation in Europe, have expressed concern over the way the vote was carried out.
Turkey's minister for European Union affairs lashed out at the EU's "unacceptable" appeal for a probe into alleged irregularities in the referendum on boosting President Recep Tayyip Erdogan's powers. Sigmar Gabriel also called on the government in Ankara not to distance itself from Europe.
Observers from the Organisation for Security and Cooperation in Europe (OSCE) and the Council of Europe said Sunday's referendum had been an uneven contest.
Erdogan, meanwhile, called the referendum "the most democratic election. ever seen in any Western country" and admonished the OSCE monitors to "know your place".
President Trump sprang ahead of Western leaders by congratulating Turkey's hardline president on his controversial referendum victory - just as an official British observer denounced it as "rigged".
In Istanbul, hundreds of "no" supporters demonstrated in the streets on Monday, chanting "thief, murderer, Erdogan" and banging pots and pans.