Ukraine and its Western allies have rejected the votes as illegitimate and neither free nor fair, saying they will have no binding force.
A look at the referendums and their potential implications:
WHY ARE THE REFERENDUMS HAPPENING?
The Kremlin has used this tactic before. In 2014, it held a hastily called referendum in Ukraine's Crimea region that also was denounced by the West as illegal and illegitimate. Moscow used the vote as a justification to annex the Black Sea peninsula in a move that was not recognized by most of the world.
On Tuesday, authorities in the separatist Luhansk and Donetsk regions that make up Ukraine's eastern industrial heartland known as the Donbas abruptly announced that referendums on joining Russia would be held starting Friday. Moscow-backed officials in the Kherson and Zaporizhzhia regions in the south also called votes.
The moves followed months of conflicting signals from Moscow and separatist officials about the referendums that reflected the shifts on the battlefield.
During the summer, when the Kremlin hoped for a quick capture of all of the Donbas region, local officials talked about organizing the votes in September.
Russian troops and local separatist forces have taken control of virtually all of the Luhansk region, but only about 60% of the Donetsk region. The slow pace of Russia's offensive in the east and the Ukrainian push to reclaim areas in the Kherson region made officials in Moscow talk about delaying the votes until November.
The Kremlin's plans changed again after a lightning Ukrainian counteroffensive this month forced Russian troops to retreat from broad swaths of the northeastern Kharkiv region and raised the prospect of more gains by Kyiv's forces.
Observers say that by moving quickly to absorb the captured territories into Russia, the Kremlin hopes to force Ukraine to halt its counteroffensive and accept the current areas of occupation or face devastating retaliation.
WHAT IS HAPPENING IN THE REGIONS WHERE THE VOTE WILL OCCUR?
The 2014 vote in Crimea was held under the close watch of Russian troops shortly after they had overtaken the peninsula, where most residents were pro-Moscow.
Separatists who have controlled large chunks of the Donbas since 2014 have long pushed for joining Russia and have shown little tolerance for dissent. When the rebellion erupted there, the separatists quickly organized referendums in which a majority voted to join Russia, but the Kremlin ignored the outcome.
The two regions declared their independence from Ukraine weeks after Crimea's annexation, triggering eight years of fighting that President Vladimir Putin used as a pretext to launch an invasion in February to protect their residents.
In the southern regions, which were occupied by Russian troops in the opening days of the invasion, anti-Russian sentiments run strong. Hundreds of pro-Kyiv activists have been arrested, with many alleging they were tortured. Others were forcibly deported, and tens of thousands fled.
Since Russian forces swept into the Kherson region and part of the Zaporizhzhia region, Moscow-appointed authorities there have cut off Ukrainian TV broadcasts, replacing them with Russian programming. They have handed out Russian passports to residents, introduced the ruble and even issued Russian license plates to pave the way for their incorporation into Russia.
Moscow-appointed administrations have come under frequent attacks by members of Ukrainian resistance movement, which has killed local officials, bombed polling stations and other government buildings, and helped the Ukrainian military target key infrastructure.
WHAT IS BEING SAID ABOUT THE LEGITIMACY OF THE VOTE?
The five-day voting process will take place in the absence of independent monitors and offer ample room for rigging the outcome.
When the referendums were announced earlier this week, the West immediately questioned their legitimacy. U.S. President Joe Biden and German Chancellor Olaf Scholz referred to them as shams, and French President Emmanuel Macron said they would have ``no legal consequences.''
Ukrainian President Volodymyr Zelenskyy also called them ``noise'' to distract the public.
HOW IS RUSSIA'S MILITARY MOBILIZATION RELATED?
A day after the referendums were announced, Putin ordered a partial mobilization of reservists to bolster his forces in Ukraine, and he also declared he was ready to use nuclear weapons to fend off any attacks on Russian territory.
The Defense Ministry said the mobilization _ Russia's first since World War II _ is intended to call up about 300,000 reservists with previous military experience. Observers noted, however, that Putin's decree is broad enough to allow the military to swell the numbers if needed. Some reports suggest the Kremlin's goal is amassing 1 million men, in a secret part of the decree.
The Kremlin long has shunned taking such a deeply unpopular move, wary of fomentin discontent and eroding Putin's support base.
The latest Ukrainian counteroffensive exposed Russia's inability to control the 1,000-kilometer (over 600-mile) front line with its current limited force of volunteers. Military experts say it will take months to make the newly called-up reservists ready for combat.
HOW IS PUTIN'S NUCLEAR THREAT RELATED?
As Putin struggles for ways to avoid new humiliating defeats, he signaled his readiness Wednesday to use nuclear weapons to protect the country's territory _ a blunt warning to Ukraine to stop pressing its offensive into the regions now set to become part of Russia.
Observers saw Putin's threat as an effective ultimatum to Ukraine and its Western backers to freeze the conflict or face a potential escalation all the way to a nuclear conflict.
While Russian military doctrine envisages using atomic weapons in response to a nuclear attack or aggression involving conventional weapons that ``threatens the very existence of the state,'' Putin's statement further lowered the threshold for their use.
Dmitry Medvedev, deputy head of Russia's Security Council chaired by Putin, amplified the president's threat Thursday, saying that after absorbing the four Ukrainian regions, Moscow could use ``any Russian weapons, including strategic nuclear weapons'' to defend them.
The mention of strategic nuclear forces, which include intercontinental ballistic missiles and long-range bombers, sent a warning that Russia could target not only Ukraine but also the U.S. and its allies with nuclear weapons in case of an escalation.
Zelenskyy dismissed the nuclear threats as bluster and vowed to free all occupied territories.