Rebel leader Alexander Zakharchenko, prime minister of the self-proclaimed Donetsk People's Republic and a mining electrician, won over 81 per cent of the votes cast, the rebels' exit polls said.
The election has worsened the standoff between Russia and the West.
The US and European Union termed the election illegitimate but the Russian side said it would recognise the result. This deepened the crisis that started after the loss of Viktor Yanukovych, the Russia-backed president, in February.
Poroshenko said the elections held in eastern Ukrainian rebel region of Donetsk was a farce held under the barrels of military might. He said the polls violated the Minsk Protocol of September 5 to which Russia is a party. The protocol was signed to end fighting between Ukrainian forces and the separatists.
The separatists ran over parts of the Donetsk and Luhansk regions after fighting occurred in eastern Ukraine for months ending with the Minsk agreement.
Rebel leaders said that as independent states, they were not required to observe Ukrainian law, and hence did not participate in Ukraine's national elections held last week.
People were seen buying carrots, potatoes and cabbages while waiting in queue in front of polling stations.