Turkey voted on Sunday to switch to a presidential system, greatly increasing Erdogan's powers.
The statement did not say whether Mr Trump had raised independent reports of voting irregularities during the Turkish referendum or the government's heavy-handed tactics in the weeks leading up to it, when the country was under a state of emergency.
"The legal framework, which is focused on elections, remained inadequate for the holding of a genuinely democratic referendum", the monitors said in a joint statement. The OSCE also said the vote did not meet standards set by the Council of Europe.
He made the comments at news conference in Ankara.
ODIHR mission head Tana de Zulueta also noted that people forced to flee their homes in areas of the southeast affected by security operations faced difficulty in voting.
The opposition have been protesting against a decision by the High Electoral Board to accept ballots without an official stamp on them. Official results are due to be announced in the next 12 days.
"Never mind the narrow victory, in fact we were expecting something around 60 percent, but it's still a victory and we are rejoicing it for the wealth and peace of our people", said Ecel.
He has also been at the center of global affairs, commanding NATO's second-biggest military on the border of Middle East war zones, taking in millions of Syrian refugees and controlling their further flow into Europe.
Critics accuse Erdogan of steering Turkey towards one-man rule.
And on Sunday, he said that if it were to be rejected by the opposition, he could put the matter to another referendum.
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So a flight attendant asked them if they had been upgraded, and when they said no, asked them to return to their assigned seats. Because the plane was only half-full, the couple chose to sit in a pair of seats three rows closer to the front of the cabin.
Deputy chairman of the People's Republican Party (CHP), Bulent Tezcan, said the party had received complaints from many regions that people had been unable to vote in privacy and said that some ballots were counted in secret. But Erdogan said he would sign it "without hesitation". About 100,000 people have been fired from their jobs in the crackdown that followed on supporters of a US -based Islamic cleric and former Erdogan ally whom the president blamed for the attempted putsch.
The changes could keep him in power until 2029 or beyond, making him easily the most important figure in Turkish history since state founder Mustafa Kemal Ataturk built a modern nation from the ashes of the Ottoman empire after World War One.
Manfred Weber, leader of the European People's Party (EPP), tweeted: "No matter the result: with his referendum President Erdogan is splitting his country".
"Power would be more concentrated under the presidency".
Response from Europe, which has had increasingly strained ties with Turkey, was cautious, awaiting the judgment of global observers later on Monday.
"Efforts to cast a shadow on the result of the vote by spreading rumors of fraud are futile and in vain", Yildirim said.
Just ahead of the final results, German Foreign Minister Sigmar Gabriel said, "We'd be well advised to keep calm and to proceed in a level-headed way".
The 18 constitutional amendments that will come into effect after the next election, scheduled for 2019, will abolish the office of the prime minister and hand sweeping executive powers to the president. That was a smaller mandate than the decisive result for which he and his ruling AK Party had aggressively campaigned.
The new system takes effect at the next election, now slated for 2019.
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