Some 90% of voters in a key industrial region in eastern Ukraine have come out in favour of sovereignty, pro-Russian insurgents said.
The rebels were announcing preliminary results of a twin referendum that is certain to deepen the turmoil in the country.
Roman Lyagin, election chief of the self-styled Donetsk People's Republic, said about 75% of the Donetsk region's three million or so eligible voters cast ballots, and the vast majority backed self-rule.
With no international election monitors in place, it was all but impossible to verify the insurgents' claims. The preliminary vote count was announced just two hours after the polls closed in an election conducted via paper ballots.
A second referendum organised by pro-Russian separatists was held in eastern Ukraine's industrial Luhansk region, but no immediate results were released.
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Ukraine's central government and the West had condemned the balloting as a sham and a violation of international law, and they have accused Moscow of orchestrating the unrest in a possible attempt to grab another piece of the country weeks after the annexation of Crimea.
In a statement released on Monday, Ukraine's acting president Oleksandr Turchynov said: "The farce, which terrorists call the referendum, will have no legal consequences except the criminal responsibility for its organisers."
In pictures: The Ukraine crisis
In pictures: The Ukraine crisis
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Ukrainian soldiers from the 'Azov' battalion guard their base in Mariupol, Donetsk
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Children were transferred from orphanages in Donetsk and Makeyevka to escape the fighting between the Ukrainian army and pro-Russian separatists
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Ukrainian servicemen captured by pro-Russian separatists sit on the ground as they are assigned to clean a street in Snizhne in the Donetsk region
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A view inside the Youth Culture Centre destroyed by pro-Russian separatists in Donetsk
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A woman holds a portrait of her dead son as she speaks during a rally in front of Ukrainian President Petro Poroshenko's office in Kiev
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A boy ascends the stairs of a bomb shelter after the shelling in the Petrovskiy district in Donetsk
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A medic looks at thirty coffins prepared for the funerals of pro-Russian rebels killed during heavy fighting at Donetsk airport, outside a Donetsk morgue
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Relatives mourn near the coffin of Mark Zverev, a taxi driver shot dead during clashes at the Donetsk airport between Ukrainian troops and the pro-Russian rebels, during his funeral in the village of Grabari on the outskirts of Donetsk
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Medical workers carry a wounded pro-Russian militant on a stretcher after armed clashes occured between pro-Russian gunmen and Ukrainian troops in Slavyansk
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A member of a newly-formed pro-Russian armed group called the Russian Orthodox Army mans a barricade near Donetsk airport
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Black smoke rises from a shot down Ukrainian Army helicopter outside Slovyansk
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A woman is overcome with emotion as she looks at blood stains and damage around a wrecked truck of supporters of the self-proclaimed 'Donetsk People's Republic' on road leading to the Donetsk International Airport
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A local man looks at damage near a wrecked truck of supporters of the self-proclaimed 'Donetsk People's Republic' on road leading to the Donetsk International Airport
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A bloodstained icon of Jesus is seen among blood soaked shattered glass atop a wrecked truck near the Donetsk airport
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A family member attends a funeral for Olga Prokhorenko (60) who was killed by shrapnel after Ukrainian government forces shelled their location, during the funeral in Slovyansk
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A woman reacts after seeing the lifeless body of a man killed by shrapnel following a shelling from Ukrainian government forces in Slovyansk
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Pro-Russian militants take position on the roof of the international airport of the eastern Ukrainian city of Donetsk
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A woman embraces a pro-Russian separatist from the "East" battalion during a rally in the eastern city of Donetsk
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A Ukrainian helicopter Mi-24 gunship fires its cannons against rebels at the main terminal building of Donetsk international airport
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A pro-Russian gunman changes his position near the airport, outside Donetsk
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A pro-Russian gunman aims his weapon near the airport, outside Donetsk. Ukraine's military launched air strikes against separatists who had taken over the airport in the eastern capital of Donetsk in what appeared to be the most visible operation of the Ukrainian troops since they started a crackdown on insurgents
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Pro-Russians ride on a truck in Donetsk. A convoy of an armored personnel carrier and seven trucks carrying several hundred heavily armed men drove through central Donetsk and gunmen got out of the trucks, stood to attention and gave shots in the air in jubilation as a crowd of several thousand supporters cheered them and chanted: "Heroes!"
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An elderly woman leans against the chest of a pro-Russian gunman in Lenin square in Donetsk
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Pro-Russian militants guard a psychiatric hospital after shell explosions during the fighting between pro-Russian militants and the Ukrainian army, in Semyonovka village, outside Slavyansk
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Ukrainian troops stand guard at a checkpoint on the road near the eastern city of Izum, Donetsk
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Members of the Democratic Alliance party take part in a performance in front of the French embassy in Kiev. Protesters set up a pool with the blood of animals and models of warships in a protest against French plans for the sale of two Mistral helicopter carriers to the Russian Navy
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A man carries a portrait of pro-Russian activist Vadim Hudich, who was killed in a shooting incident at the head of a funeral procession in the eastern Ukranian town of Krasnoarmeisk
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A voter is seen inside a voting booth at a polling station during the referendum on the status of Donetsk region in the eastern Ukrainian city of Donetsk
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Members of a local election commission sort ballots as they start counting votes of referendum on the status of Luhansk region in Luhansk
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Members of a local election commission empty a ballot box as they start counting votes of the referendum on the status of Donetsk region in Donetsk
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Members of a local election commission empty ballot boxes as they start counting votes after a referendum, at a polling station in Lugansk
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Local residents watch as others give first aid to a man who was shot in the leg by Ukranian militia in the village of Krasnoarmisk. Eyewitness said that Ukranian militia tried to stop the referendum voters briefly taking the City Hall of Krasnoarmisk, where unarmed pro-Russian supporters were gathering. Reportedly the Ukranian militia came out out the building and started shooting at the people, killing at least one man and leaving at least two others injured
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Members of a local election commission count votes after a referendum organized by the so-called Donetsk People's Republic members at a polling station in Donetsk
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Voters visit a polling station to take part in the referendum on the status of Donetsk
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An Armoured Personnel Carrier (APC) with a Russian flag drives through the center of Slaviansk during the day of referendum organized by the so-called Donetsk People's Republic members in Slavyansk
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A statue of Lenin is placed in front of a pro-Russian barricade on the outskirts of the eastern Ukrainian town of Slavyansk
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A feamle veteran (C) sings during 'Victory Day' celebrations in Donetsk
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People stand near the burning Mariupol police station
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Ukrainian soldiers stand guard beside an armoured personnel carrier at a checkpoint in Mariupol. Ukrainian forces seized the rebel-held city hall in the eastern port city of Mariupol, driving out pro-Russian activists, then withdrew, making no attempt to hold onto the building
The results of the two referendums could hasten the break-up of the country and worsen what is already the gravest crisis between the West and Russia since the end of the Cold War.
Although the voting in the two regions with a combined population of 6.5 million appeared mostly peaceful, armed men identified as members of the Ukrainian national guard opened fire on a crowd outside the town hall in Krasnoarmeisk, and an official with the region's insurgents said people were killed. It was not clear how many.
The bloodshed took place hours after dozens of armed men shut down the voting in the town.
The shooting starkly demonstrated the hair-trigger tensions in the east, where pro-Russian separatists have seized government buildings and clashed with Ukrainian forces over the past month.
Even before the results were announced, Ukraine's foreign ministry called the twin referendums a "criminal farce". The US and other Western governments said they would not recognise the outcome.
Earlier in the day, the head of the referendum organisers in Donetsk said the ultimate status of the region would be discussed later and would include the possibility of secession or annexation by Russia.
"We are just saying to the world that we want changes, we want to be heard," election commission head Roman Lyagin said.
The violence in Krasnoarmeisk, about 20 miles (30km) from the regional capital, Donetsk, came hours after armed men, one of whom said they were from the national guard, put a stop to the voting and took control of town hall.
In the evening, more armed men arrived in a van, and a scuffle broke out with people gathered around the building. Then the men fired shots.
An Associated Press photographer who witnessed the shooting said two people lay motionless on the ground. Insurgent leader Denis Pushilin was quoted by the ITAR-Tass news agency as saying there were an unspecified number of deaths.
Over the past few weeks, the Ukrainian government and the West have accused Russia of trying to destabilise the country or create a pretext for another invasion.
Russia - which annexed Ukraine's Crimean Peninsula just days after voters there approved secession in a March referendum - has rejected the accusations.
Russian president Vladimir Putin had asked the organisers of the latest referendums to delay the vote in an apparent attempt to ease the crisis. The insurgents refused.
At one polling station at a school in Donetsk, turnout was brisk in the first hour of voting. All voting slips that could be seen in the clear ballot boxes showed that self-rule had been selected.
Most opponents of sovereignty appeared likely to stay away from the polls rather than risk drawing attention to themselves.
Darya, a 25-year-old medical worker who would not give her last name, said she saw no point in casting a ballot, since the vote had no legal force.
"There were no notices about this referendum anywhere, about where and when it was happening," she said. "In any case, it is not valid, so there was no reason to take part."
There were no immediate signs of any outright intimidation by pro-Russian forces, and insurgents near the polls were not wearing their usual balaclavas.
The haphazard nature of the referendums was in full display at Spartak, a leafy village on the fringes of Donetsk.
Villagers were unable to vote for about three hours after the polls opened because election officials failed to bring a ballot box. Finally, an election organiser arrived with a voting urn crudely fashioned from cardboard boxes and sealed with tape.
Most present said they were voting in favour of autonomy and against the interim government headed by acting president Oleksandr Turchynov. One said she would not take part in a nationwide presidential election set for May 25.
"I don't agree with what is happening in the country. And I want some changes for the better. What is happening on May 25 is not honest, truthful or in our best interests. And that is why I am voting today," said Irina Zelyonova, 30, cradling her baby in her arms.
Mr Turchynov and Ukraine's caretaker government came to power in February following the ouster of Kremlin-friendly president Viktor Yanukovych after months of protests in Kiev.
Moscow and many in Ukraine's east have accused the new government of intending to trample the rights of eastern Ukraine's Russian-speakers.
More than 30 people have been reported killed since Ukrainian forces began trying to retake some eastern cities from the insurgents.