Moscow is seeking to annex areas its forces have occupied during nearly seven months of war.
The voting overseen by Moscow-installed authorities, is expected to return a majority in favour of joining Russia.
Ukraine and the West have denounced the referendums as a sham and an illegitimate step toward annexation. A similar vote took place in 2014 before Russia annexed Ukraine’s Crimean Peninsula, a move that most of the world considered illegal.
Election officials planned to bring ballots to people’s homes and setting up makeshift polling stations near residential buildings during the first four days of voting, according to Russian-installed officials.
Polls also opened in Russia, where refugees and other residents of the occupied regions could cast their votes.
Denis Pushilin, the separatist leader of Moscow-backed authorities in the Donetsk region, called the referendum taking place there “a historical milestone.”
Vyacheslav Volodin, the speaker of Russia’s lower house of parliament, the State Duma, addressed the occupied regions Friday in an online statement, saying: “If you decide to become part of the Russian Federation — we will support you.”
However, leaders in the West including French President Emmanuel Macron and Prime Minister Liz Truss have dismissed the referanda as having no credibility.
Ukrainian President Volodymyr Zelenskyy only briefly mentioned the “sham referenda” in his nightly address in which he switched from speaking in Ukrainian to Russian to directly tell Russian citizens they were being “thrown to their deaths.”
“You are already accomplices in all these crimes, murders and torture of Ukrainians,” he said. “Because you were silent. Because you are silent. And now it’s time for you to choose. For men in Russia, this is a choice to die or live, to become a cripple or to preserve health. For women in Russia, the choice is to lose their husbands, sons, grandchildren forever, or still try to protect them from death, from war, from one person.”
The voting takes place against the backdrop of ongoing fighting in Ukraine with Russia recently losing territory it had seized. In response Vladimir Putin has announced that up to 300,000 reservists will be sent into the fight.