The scathing report, issued on Saturday - a day before the elections - prompted a furious response from Kazakh officials.
The Organisation for Security and Co-operation in Europe (OSCE) said the Kazakh government had not fulfilled most of the 24 recommendations made in an OSCE report on preparations for the election.
Urdur Gunnarsdottir, a spokeswoman, said in Astana, the Kazakh capital: "No they have not. They have implemented a few of the recommendations."
However, she said "it's still possible to have a good election".
Marat Sarsembayev, a key official from the Kazakh Central Elections Commission, said the OSCE had "caused bewilderment".
"The observers from the OSCE non-objectively and unprofessionally broadened their demands," he told a press conference.
"All 24 recommendations which were presented in the report ... have been fulfilled by our side."
Charges against OSCE
He said the OSCE had refused to listen to election officials' explanations and had shown "lack of objectivity and one-sidedness in the evaluation of the process, including of the political process in our country".
Gunnarsdottir described Sarsembayev's comments as "quite outrageous".
"Most, if not all of what they said, we would refute. This came out of the blue. We've had very good co-operation with them."
Nursultan Nazarbayev is expected to retain the presidency in Sunday's vote, against four challengers.
Kazakhstan has never held an election judged to meet Western standards, and the election campaign was marred by accusations of harassment against opposition activists and the media.
The OSCE, the most influential election group, is to issue a report on the election's conduct on Monday.