People in the breakaway republics of Luhansk and Donetsk in eastern Ukraine, as well as the Russian-controlled parts of Kherson and Zaporizhia, voted on the fifth and final day of referendums on joining Russia on Tuesday.
Leader of the Luhansk People's Republic, Leonid Pasechnik, cast his ballot on Tuesday as voting, which began on Friday, is due to end later in the day.
He said he had voted for joining Russia, after seven months of simmering war between Moscow and Kiev.
It comes as Kiev warned voters of facing charges of treason.
"We have lists of names of people who have been involved in some way," Ukraine's presidential adviser Mikhailo Podolyak said in an interview with Swiss newspaper Blick.
"We are talking about hundreds of collaborators. They will be prosecuted for treason. They face prison sentences of at least five years."
However, Podolyak said Ukrainians who were allegedly forced to vote would not be punished.
Russian President Vladimir Putin has backed the referendums, which were slammed by Western nations as a “sham” who vowed not to recognize their results.
The voting takes place as fighting rages on between Ukrainian and Russian troops.
Russia launched the military operation in the former Soviet republic following Kiev’s failure to implement the terms of the 2014 Minsk agreements and Moscow’s recognition of the breakaway regions of Donetsk and Luhansk.
At the time, Putin said one of the goals of what he called a “special military operation” was to “de-Nazify” Ukraine.
The United States and its European allies have imposed waves of economic sanctions against Moscow while supplying large consignments of heavy weaponry to Kiev despite Russian warnings.
Moscow has been critical of weapons supplies to Kiev by Washington and its Western allies, warning it will only prolong the conflict.
In 2014, Russia held a referendum in Ukraine’s Crimea, when 97 percent of voters favored joining the Russian state amid condemnation by Kiev and its Western sponsors.