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.
Mr 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.
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 Mr 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.