Turkey's ruling AK Party set out plans on Wednesday for President Tayyip Erdogan to gradually take back the party reins, in a sign it would begin implementing changes approved in Sunday's referendum despite opposition attempts to annul it.
"Calling people to the street is wrong and is outside the line of legitimacy", Yildirim said, adding, "we expect the main opposition party's leader to act more responsibly".
The election observer mission by the Organization for Security and Co-operation in Europe (OSCE), which scrutinized the referendum, issued a statement on its findings on April 17, 2017, describing the "unlevel playing field" in the period before the vote.
Voting was already underway on Sunday when the electoral authority announced it would start accepting unstamped ballots - a decision that the opposition said should invalidate the results.
Hundreds of people lined up outside election board offices in Ankara and Istanbul to submit petitions requesting the board reverse its pronouncement.
The European Union also urged a probe into the poll fraud claims after worldwide observers voiced concerns, but both US President Donald Trump and Russian President Vladimir Putin called Erdogan to offer congratulations.
The queues formed as the main opposition party was scheduled to formally request that the electoral authority annul the referendum over the ballots lacking the official stamp.
"We demand the cancellation of this referendum", Tezcan said.
There have been daily street protests in anti-Erdogan neighbourhoods in Istanbul since Sunday's referendum, which the opposition claims was marred by blatant vote rigging.
Earlier, 19 people were detained for allegedly using the results of a constitutional referendum as an "excuse" to organize "unauthorized demonstrations", official Anadolu news agency reported.
"Any move to reintroduce the death penalty would be another disastrous step away from human rights norms for Turkey", Williamson said. The Commission said that any legislation bringing back the death penalty to Turkey, as pledged by Erdogan during his campaign, would certainly end Ankara's European Union membership bid.
On Monday, Erdogan renewed suggestions that Turkey could hold referendums on its bid to join the European Union and on reinstating the death penalty. The narrow 51.4 percent margin of victory also revealed doubts among Turkish voters about the constitutional changes.
The brief statement - issued on Monday evening - underlines the United States need for Turkey's support for the Trump administration's new policy of increasing pressure on the Syrian government, as there's only one reference to the April 16 referendum in Turkey and three to the crisis in Syria.
Opposition parties filed formal requests Tuesday to void the results over voting irregularities, particularly an electoral board decision to accept ballots without official stamps, as required by Turkish law.
Two worldwide observer missions said the referendum campaign had been conducted in an unfair environment in which opposition voices were suppressed.
Relations have further soured since Erdogan accused the German and Dutch governments of acting like Nazis after they banned referendum campaign rallies by Turkish officials.