Alexander Zakharchenko -- prime minister of the self- proclaimed Donetsk People's Republic -- was estimated to get 81 percent of the presidential vote, according to a rebel exit poll released after the ballot ended.
The election -- along with another in the neighbouring rebel Lugansk region -- has been backed by Russia and billed as a way of bringing a degree of legitimacy to the rebel control over two main industrial cities seized in an armed uprising.
Ukrainian President Petro Poroshenko blasted the ballot as a "farce that is being conducted under the threat of tanks and guns" and warned Moscow not to follow through on its intention to recognise the result.
The run-up to the vote saw a spate of shelling by rebels of government positions across the conflict zone, where according to UN figures more than 4,000 people have died since fighting started around seven months ago.
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Fuelling concern of fresh fighting, Kiev's military also claimed it had detected "intensive" movement of troops and equipment from Russian territory.
The reported deployments, which would constitute a major escalation of Russian involvement, could not be verified.
AFP journalists in rebel-controlled Donetsk saw a column of about 20 trucks, some carrying heavy anti-aircraft guns, heading in the direction of the government-held airport, although there was a notable drop in fighting during the polling.
The rebels -- who deny being helped by Russia, but boast an arsenal that includes anti-aircraft missiles, tanks and heavy artillery -- have threatened to expand their offensive to the Azoz Sea port city of Mariupol.
But residents of rebel-held areas spoke of their hatred for the government in Kiev and their desire for the war to end.