The Kremlin-orchestrated referendums, which have been widely denounced by Ukraine and the West as shams without any legal force, are seen as a step toward annexing the territories by Russia.
The votes are being held in the Luhansk, partly Russian-controlled Zaporizhzhia and Donetsk regions.
In Kherson, which is almost fully controlled by Moscow, the balloting was also expected to get underway on Friday morning.
The vote, which asks residents if they want their regions to be part of Russia, is certain to go Moscow’s way.
Latest Defence Intelligence update on the situation in Ukraine – 23 September 2022
Find out more about the UK government’s response: https://t.co/Bot9jAVNWg
— Ministry of Defence 🇬🇧 (@DefenceHQ) September 23, 2022
That would give Russia the pretext to claim that attempts by Ukrainian forces to regain control are attacks on Russia itself, dramatically escalating the seven-month-old conflict.
The referendums follow Russian President Vladimir Putin’s order of a partial mobilisation, which could add some 300,000 Russian troops to the fight.
The balloting will continue for five days until Tuesday.
According to Russian-installed officials in the occupied regions, who cited safety reasons, during the first four days election officials will be bringing ballots to people’s homes and setting up makeshift polling stations near residential buildings.
Tuesday will be the only day when the voters will be invited to come to regular polls.
Polls also opened in Russia, where refugees from the occupied regions can cast their votes.
Denis Pushilin, separatist leader of Moscow-backed authorities in the Donetsk region, called the referendum on Friday “a historical milestone”.