Officials in Moscow warned against any protests during the vote, after calls from the opposition for anti-Putin demonstrations at midday on Sunday.

Putin is set to extend his rule by another six years this weekend in a presidential election the Kremlin says will show that the country is fully behind his assault on Ukraine.

Ahead of the vote, Kyiv has ramped up its aerial bombardment of Russian regions just across their shared border.

And the Russian national guard said it was fighting off attacks from pro-Ukrainian militias in Kursk, the latest in a string of border clashes.

"I am convinced: you realise what a difficult period our country is going through, what complex challenges we are facing in almost all areas," Putin said in an address to Russians on the eve of the vote.

"And in order to continue to respond to them with dignity and successfully overcome difficulties, we need to continue to be united and self-confident."

All of Putin's major critics are dead, in prison or in exile, and authorities blocked the few genuine competitors who tried to stand in the March 15-17 contest.

Alexei Navalny, Putin's most high-profile opponent over the last decade, died in February in an Arctic prison colony. He was serving 19 years for "extremism", a sentence widely seen as retribution for his campaigning against the Kremlin leader.

On Thursday, Moscow prosecutors warned against protests during the election.

"The organisation of and participation in these mass events are punishable by virtue of the legislation in place," they said in a statement posted on Telegram.

Navalny's widow, Yulia Navalnaya, has called for rallies outside polling stations on Sunday, the final day of voting.

Kyiv has this week launched some of its most significant aerial attacks since the start of the two-year conflict.

The governor of Russia's Belgorod region, Vyacheslav Gladkov, said Thursday that at least three separate waves of aerial attacks had killed two people, wounding several others.

He accused Ukraine of trying to "sow panic, distrust, anger and resentment, in order to break the unit of our society".

Shopping centres in the region were closed on Thursday and Gladkov urged residents to delay taking trips outside "to save your health and your life".

Pro-Ukrainian paramilitaries also claimed to be escalating attacks and incursions in Russian border regions.

In a joint statement Thursday, three pro-Kyiv volunteer groups -- claiming to consist of anti-Kremlin Russians who have taken up arms -- called on authorities to evacuate civilians from the regions of Belgorod and Kursk.

"Civilians should not suffer from the war and any casualties in the process of fighting will be on the conscience of Starovoit and Gladkov," they added, referring to the regions' governors.

Russia has denied militias' claims to have gained ground.

The national guard said Thursday its units had beaten back one such attack near the village of Tyotkino in the Kursk region.

The defence ministry said it had fended off another by Ukrainian forces trying to enter the Belgorod region via the village of Spodariushino, without saying when the clash had happened.

It published video images showing air strikes on what it said was a Ukrainian sabotage group. The fighting come just hours ahead of polls opening in Russia's Far East for the presidential election.

Victory will allow Putin to stay in the Kremlin until at least 2030, a longer spell in power than any Russian leader since Catherine the Great in the 18th century.

He called on Russians to use the election to show their unity behind his leadership.

"We have already shown that we can be together, defending the freedom, sovereignty and security of Russia," he said in a video message, flanked by flags of the Russian tricolour and the president's state insignia.

"Today it is critically important not to stray from this path," he said.

Early voting is already underway in occupied territories of Ukraine. The vote will also take place in Crimea, the peninsula annexed by Moscow in 2014 -- a move that most of the international community refuses to recognise.

In the Russian-controlled Ukrainian city of Mariupol, election officials on Thursday opened pop-up polling stations at small tables in the street and on the hoods of cars.

Banners were unfurled sporting a red, white and blue "V" logo -- an army symbol used as a sign of support for the military offensive.

Ukraine's foreign ministry on Thursday urged the international media and public figures "to refrain from referring to this farce as 'elections' in the language of democratic states".

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