The interim Kyrgyzstan government today went ahead with a constitutional referendum, just two weeks after an outbreak of ethnic violence in which at least 2,000 people, most of them ethnic Uzbeks, were killed and tens of thousands left homeless.

The country's interim leader, Rosa Otunbayeva, urged Kyrgz citizens to take part in the ballot, designed to legitimise the temporary government and replace the country's abuse-prone presidential system with a more European-style parliamentary democracy.

But international observers and human rights groups have questioned the timing of the referendum, which takes place against a backdrop of the worst ethnic violence in Central Asia for two decades.

They also warned that the vote could exacerbate divisions between the north and south of the country and hasten its break-up, or lead to civil war.

Voting in Osh, the city at the centre of the violence, Otunbayeva admitted Kyrgyzstan was "on the brink of great danger".

She added: "The results of this referendum will show that the country is united and that the people are one. It will stand strong on its own feet and move forward."

In a TV address yesterday, Otunbayeva also said the ballot would enable Kyrgyzstan to make a clean break with the previous autocratic regime.

The current opposition-led government took power in April after violent street protests in the capital, Bishkek, forced the president, Kurmanbek Bakiyev, to flee. Bakiyev is now in exile in Belarus.

He told the BBC: "So many people have died, there are hundreds of thousands of refugees, and in this situation they still want to have a referendum? It's simply unjust. What referendum?"

But Otunbayeva said: "You should take part in the referendum to get rid of the Bakiyev constitution, which threatens the return of the mafia-clan pyramid of power."

She added that Kyrgyzstan should not skip a "historical opportunity" to go down a "democratic route" free of bribery, pressure and cheating.

Voters are being asked whether they approve of a new constitution.

If it is adopted, parliamentary elections will be held every five years, with a largely ceremonial president elected for a six-year term.

Parliamentary elections are due to be held in October, with Otunbayeva, a former diplomat and ambassador in London, declaring she will stay in power until 2011 before stepping aside.

Riots erupted in Osh on 10 June, spreading to neighbouring Jalalabad.

An apparent dispute between Uzbek and Kyrgyz youths escalated into what Uzbeks said was genocide.

Survivors said Kyrgyz soldiers using armoured personnel carriers, and acting with the apparent complicity of the army, police and local administration, opened fire on men, women and children. A civilian Kyrgyz mob then looted and burned Uzbek homes.

The UN estimates that the violence displaced 400,000 Uzbeks in the south of the country, with 75,000 fleeing across the nearby border into Uzbekistan.

Most have now returned and are staying in makeshift accommodation, but rights groups said the situation remained tense, with both groups having retreated into ethnic near-homogenous areas and fearful of further attacks. Uzbeks have also complained of harassment from Kyrgyz security forces.

Fyodor Lukyanov, the editor in chief of Russia in Global Affairs, said: "I can't imagine how you can have a referendum in a situation of de facto civil war.

"The local authorities hope to legitimise themselves. But if the southern part will not take part in the referendum it will effectively split the country. I'm afraid the dangers for Kyrgyzstan have just started."

Russia and the US both have airbases in the north of the country, but have been reluctant to commit peacekeeping forces.

Human Rights Watch and the International Crisis Group last week called for an international stabilisation mission to Kyrgyzstan to provide security for refugees, prevent more violence and ensure the delivery of humanitarian aid.

"The government's decision to proceed with the referendum and the return of thousands of refugees and internally displaced people to almost uninhabitable areas makes the situation even more volatile," Human Rights Watch said, noting that many Uzbek refugees had no documents or ID.

The International Crisis Group said there was now a real risk of "large-scale ethnic violence" spreading across the multinational Ferghana valley, which is divided between Uzbekistan, Kyrgyzstan and Tajikistan.