Turkey voted on Sunday in a referendum on granting President Recep Tayyip Erdogan extra powers that was won by the 'Yes' camp but disputed by the opposition.
"The referendum took place on an unlevel playing field and the two sides of the campaign did not have equal opportunities," said Cezar Florin Preda of the joint mission of the OSCE Office for Democratic Institutions and Human Rights (ODIHR) and the Parliamentary Assembly of the Council of Europe (PACE).
"Late changes in counting procedures removed an important safeguard," said Mr Preda, who headed the PACE delegation, referring to the election authorities' decision to permit ballot papers without an official stamp.
"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.
WATCH: Most Turkish Australians voted 'No' in the Turkey referendum
Preda also said the fact that the referendum was held under a state of emergency imposed in the wake of last year's failed coup infringed upon a "fundamental freedom".
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 campaign rhetoric was tarnished by some officials equating 'No' sympathisers with terrorists," Ms de Zulueta said.
These contravened OSCE commitments and Council of Europe standards "regarding freedom and equality in the campaign," she said.
"Our monitoring showed the 'Yes' campaign dominated media coverage," the monitor added.
But Preda added: "It's not our role to say what is the level of fraud or (comment on) the other allegations made by the opposition."
"We are not talking about fraud and have no information on this subject," he said, noting the allegations were made by political parties.
Returning in triumph to his presidential palace in Ankara, Erdogan addressed thousands of supporters gathered outside, telling monitors who criticised the poll: "Know your place."
Erdogan told the monitors to "know your place", saying Turkey had no intention of paying any attention to the ODIHR report.
He added: "This country held the most democratic polls that have never been seen in any other country in the West."
Watch: Erdogan's 'first job' after Turkey referendum win
Trump congratulates Erdogan on referendum win
US President Donald Trump called his Turkish counterpart Recep Tayyip Erdogan on Monday to congratulate him on winning the referendum boosting his powers, the White House.
The call was first reported by Turkish state media, one day after Erdogan won over 51 percent in Sunday's vote.
"Trump called Erdogan tonight (Monday) and congratulated him on his success in the referendum," Turkish presidential sources said, quoted by the government run Anadolu news agency.
The White House in a statement later on Monday said that the two leaders discussed a range of topics in addition to the vote in Turkey.
"President Donald J. Trump spoke today with President Recep Tayyip Erdogan of Turkey to congratulate him on his recent referendum victory and to discuss the United States's action in response to the Syrian regime's use of chemical weapons on April 4th," the White House statement said.
"President Trump thanked President Erdogan for supporting this action by the United States, and the leaders agreed on the importance of holding Syrian President Bashar al-Assad accountable," the statement continued.
"President Trump and President Erdogan also discussed the counter-ISIS campaign and the need to cooperate against all groups that use terrorism to achieve their ends."
