The Istanbul Bar Association filed a criminal complaint against electoral board head Sadi Guven for "wrongful conduct" and "altering the result of the election".
The party said it planned to appeal to Turkey's constitutional court and, if it is unsuccessful there, then the European Court of Human Rights.
"This is a serious legitimacy crisis".
"We demand the cancelation of this referendum", Bulent Tezcan said, as quoted by AP.More news: Pro-pot activists plan to hand out joints outside Capitol
Turkey's state-run news agency says 19 people were detained in Istanbul for allegedly using the results of a constitutional referendum as an "excuse" to organize "unauthorized demonstrations".
Turkey's ruling AK Party had set out plans earlier Wednesday for Erdogan to gradually take back the party reins, in a sign it would begin implementing changes approved in Sunday's referendum. Turkey's main opposition party has filed a formal request seeking Sun.
But Binali Yildirim warned that calling for street protests was unacceptable. There were widespread expectations he would take over the leadership nearly immediately after the vote.
Opposition parties including the main opposition CHP and the pro-Kurdish HDP had called on the electoral board to annul Sunday's vote, which was narrowly won by the "Yes" camp, because unstamped ballot papers were included. Observers said this decision, made in the afternoon of election day, removed an important safeguard against fraud. "You can not build a change in the political system on such a controversial and unfair referendum". "You can not build a change in the political system on such a controversial and unfair referendum".More news: 9 months of sex scandals take down two Fox News icons
Kilicdaroglu said he respected the nation's will, but the decision on unsealed ballots had overshadowed the results.
"What makes any country a democracy is the security of the ballot boxes", Feyzioglu told Reuters.
Turkish President Tayyip Erdogan will meet U.S. President Donald Trump in May ahead of a meeting of the North Atlantic Treaty Organisation alliance, Turkish Foreign Minister Mevlut Cavusoglu said on Wednesday.
Worldwide election monitors, including from the Organization for Security and Cooperation in Europe, noted a series of irregularities, and said the decision to accept as valid ballots without official stamps undermined safeguards against fraud and was contrary to Turkish law.More news: New York City firefighter dies after falling several stories
"The OSCE's report has no reliability as their observations lack objectivity and are extremely partial", Cavusoglu told a news conference in Ankara.
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