A referendum which gave 75% support to sweeping constitutional reforms planned by the Kyrgyz government was plagued with violations, monitoring groups say.
Critics say President Bakiyev is tightening his grip on power
Since Sunday's vote, President Kurmanbek Bakiyev has adopted the changes and called a 16 December poll.
But observers from Europe and the US reported ballot-stuffing and said the official turnout of 80% was inflated.
The president says the reforms will give people more power, but critics say they will result in authoritarianism.
The OSCE, a European monitoring organisation, said there had been reports of a "high number of irregularities" including obstruction of observers and "massive ballot-stuffing" during the referendum.
"The observers also noted that the population had little awareness of the constitutional reform proposals that were being decided on in the referendum," said Markus Mueller, OSCE head in the capital, Bishkek.
"This fact may raise serious questions about the voter turnout of more than 80%, which was preliminarily reported by the Central Election Commission of the Kyrgyz Republic."
The US embassy in Bishkek also expressed concerns over "credible reports" of vote violations, saying the conduct "did not meet international standards".
But the election commission has denied any major irregularities.
President Bakiyev swept to power in 2005, ousting long-term leader Askar Akayev and winning a landslide victory in presidential elections.
But since then, the Central Asian country has been hampered by a political stand-off between the president and parliamentary deputies elected during the Akayev era.
The new constitution includes a proportional representation system of voting, where candidates are selected from central party lists rather than locally elected.
The president has formed his own party, Ak Zhol, and analysts say the change will help him gain a foothold in the chamber.
Other reforms to the constitution include giving parliament more power in forming the government, limiting the president's ability to dismiss parliament and raising the number of deputies.
While President Bakiyev says the changes will help to clarify the separation of powers, his critics say he is shoring up his power-base in parliament.
The former Soviet republic is strategically crucial for the US, whose only remaining military base in Central Asia lies just outside Bishkek.