Critics fear the changes will lead to autocratic one-man rule under Erdogan, but supporters say they simply put Turkey in line with France and the United States and are needed for efficient government.
The Organization for Security and Cooperation in Europe (OSCE) protested that the decision to accept unofficial ballots opened the vote to fraud, but the empowered Erdogan dismissed the OSCE assessment and described the referendum as "the most democratic election... ever seen in any Western country".
Opposition parties are filing their objections to the electoral board Tuesday for the decision to count as valid ballots that did not bear an official stamp and other alleged irregularities.
Hundreds of people lined up outside election board offices in Ankara and Istanbul to submit petitions requesting the board reverse its pronouncement. Opponents accuse him of leading a drive toward one-man rule in Turkey, a North Atlantic Treaty Organisation member that borders Iran, Iraq and Syria and whose stability is of vital importance to the United States and the European Union. Under the new system, the president will be able to appoint ministers and senior government officials, issue decrees and declare states of emergency. The head of the board said it had received many complaints that polling stations didn't have stamps and made the decision to accept the ballots after an appeal from a ruling AK Party official. "Our regret is not over the outcome of the referendum, but because of the desire to overlook clear and harsh violations of the law that have the potential to impact the results", it said.
White House press secretary Sean Spicer responded to the result of Turkey's referendum with caution and said he would withhold from commenting until a final report of election observer is complete by next week.
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Turkey's President Recep Tayyip Erdogan, delivers a speech during a rally of supporters a day after the referendum, outside the Presidential Palace, in Ankara, Turkey, Monday, April 17, 2017.
The party said he was detained on accusations of "agitating the public" by claiming the "Yes" vote was illegitimate.
"Any move to reintroduce the death penalty would be another disastrous step away from human rights norms for Turkey", Williamson said.
The referendum has also caused new friction in Turkey's relationship with the European Union, which it has long sought to join but which gave the "Yes" victory the most lukewarm welcome.
On Monday, Erdogan renewed suggestions that Turkey could hold referendums on its bid to join the European Union and on reinstating the death penalty.
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But the main opposition Republican People's Party (CHP), along with individuals and civil society groups, have challenged the result - amid concerns over unstamped ballots in particular - and called for it to be annulled.
Anadolu news agency cited a statement by Istanbul Police that said the suspects were detained early Wednesday for attempting to provoke a response similar to the mass anti-government demonstrations that happened across Turkey in 2013.
According to Turkish officials, the leaders also discussed the situation in Syria, with Trump commending Turkey's support of recent USA military strikes against the regime of dictator Bashir al-Assad.
Global monitors said the move undermined safeguards against fraud.
Donald Trump congratulates Turkey's hardline president Recep Erdogan on referendum win
Turkey's main secularist opposition said it will launch an appeal against the result this afternoon. A prosecutor will now consider whether to press charges against Guven.