International observers have backed Kyrgyzstan's referendum on a new constitution, describing it as largely transparent and peaceful.
With nearly all ballots counted, the electoral commission said voters had overwhelmingly approved the changes, which will give parliament more power.
The interim government called the vote after President Kurmanbek Bakiyev was overthrown in April.
Russia said a parliamentary system may be too weak and fan instability.
The referendum took place about two weeks after a wave of fighting between ethnic Kyrgyz and Uzbeks in the south, in which officials fear up to 2,000 people may have been killed.
Speaking on the sidelines of the G20 meeting, Russian President Dmitry Medvedev said: "I have a hard time imagining that a parliamentary republic could work in Kyrgyzstan, that it won't provoke a series of problems and encourage the rise to power of extremist forces."
He said that Kyrgyzstan faced "the threat of breaking up", adding that to avoid this "a strong, well-organised authority" was needed.
An estimated 400,000 people - many of them from the minority ethnic Uzbek community - were displaced during the recent conflict.
Although the violence has abated, the country's Uzbek and Kyrgyz populations remain deeply divided.
Despite high tensions in Kyrgyzstan, the monitors from the Organisation for Security and Co-operation in Europe (OSCE), which monitored the referendum, said voting had been peaceful.
However it said improvements were needed before upcoming parliamentary elections.
The OSCE said a high voter turnout on Sunday showed the resilience of the Kyrgyz people.
"The citizens of Kyrgyzstan turned out in large numbers to vote for a new, democratic and peaceful future for their country," said Boris Frlec, head of the observation mission.
"It is now up to all political forces to work together to... build a democratic society based on respect for human rights and the rule of law."
The OSCE said it had a limited presence in the southern cities of Osh and Jalalabad because of security concerns but that it had evidence of "participation and non-violence" there.
With ballots from nearly all precincts counted, the Central Election Commission said about 90% supported the new constitution.
Some 2.7 million people were eligible to vote, and turnout was nearly 70%, it said.
Interim leader Roza Otunbayeva called the vote historic, saying the Central Asian nation was now on the path "to a true government of the people".
She will now be inaugurated as a caretaker president and form a government.
Opposition leaders said the figures were impossibly high given the ongoing repercussions from the ethnic clashes. They had also criticised the timing of the vote as premature given the troubles.
Parliamentary elections are to be held in October and Ms Otunbayeva will remain president until the end of 2011, when she is to step aside.
The referendum was supported by the UN, the US and Russia, as a step towards restoring democracy.
Kyrgyzstan was part of the Soviet Union before its independence in 1991. Both the US and Russia have military bases in the country.