Moscow, Russia - Ukrainian bombardments killed two people and set an oil facility ablaze in Russia on Saturday, the second day of showpiece elections guaranteed to cement President Vladimir Putin's hardline rule.

Presidential polls opened this week, but voting has been marred by an uptick in fatal Ukrainian aerial attacks and a series of incursions into Russian territory by pro-Ukrainian sabotage groups.

Fresh bombardments prompted authorities to close schools and shopping centers in the Belgorod region bordering Ukraine, undermining the Kremlin's efforts to isolate Russians from its two-year conflict with Kyiv.

Putin, who cast his vote online, has vowed a harsh response to the assaults and accused Ukraine of trying to "disrupt" his bid for another six-year mandate.

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The Belgorod governor said two residents were killed and others injured on Saturday.

"A man was driving a lorry when a shell hit him, after which the vehicle crashed into a passenger bus. The people on it were not injured," Vyacheslav Gladkov wrote on social media.

"Another woman was killed in a parking lot where she and her son came to feed the dogs. Medics are fighting for her son's life," he added.

Throughout Saturday, Russia's defense ministry said it had downed rockets, missiles, and drones over Belgorod and Kursk, another border region that has seen an uptick in attacks.

Kremlin spokesman Dmitry Peskov told state media the Russian leader was being "constantly briefed" by his military leaders on the situation at the border.

Both Peskov and the defense ministry said Russian forces had fought off more attempted infiltrations by "Ukrainian militant sabotage and reconnaissance groups" overnight.

Kremlin proxy officials in the occupied Kherson region of southern Ukraine meanwhile said one person was killed and four wounded in a drone attack.

The border attacks were a concern for voters hundreds of miles away in the town of Sergiev Posad outside Moscow, famous for its ornate Orthodox monastery with golden onion domes.

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Casting her ballot from home with the help of election officials going door-to-door, 87-year-old Inessa Rozhkova said she hoped the polls would bring an end to the conflict with Ukraine.

"Can you imagine how many people died? And now our border villages are suffering. We worry for them," she said.

In a nearby polling station at a vocational school, Elena Kirsanova (68) came with her husband to vote for Putin.

"They try to scare us, but this is not a nation that can be intimidated," Kirsanova told AFP.

Putin (71) has been in power since the last day of 1999 and is set to extend his grip over the country until 2030.

If he completes another Kremlin term, he will hold power longer than any Russian leader since Catherine the Great in the 18th century.

He faces no genuine competition in the vote, having barred two candidates who opposed the conflict in Ukraine.

And his main domestic opponent, Alexei Navalny, died last month in an Arctic prison in unexplained circumstances.

The Kremlin has pitched the election as an opportunity for Russians to show they are behind Moscow's full-scale military campaign in Ukraine and Putin's anti-Western agenda.

"The actions of the West ... unites the people of Russia more," 70-year-old voter Lyubov Pyankova told AFP on Saturday in Saint Petersburg, Putin's home city.

Voting was also taking place in parts of Ukraine occupied by Russian forces – a move decried as "illegal" and a "sham" by Kyiv and Western governments.

On Saturday, state media showed soldiers and election officials collecting ballots from elderly residents in Avdiivka, the frontline city destroyed by months of fighting before being captured by Russian forces last month.

Authorities accused at least two more Russians – one in the central city of Yekaterinburg and in the western exclave of Kaliningrad – of pouring green ink into ballot boxes following a spate of arrests for similar acts and arson attacks a day earlier.

They face up to five years in prison under charges of obstructing election proceedings.

The substance being poured into the ballot boxes resembles zelyonka, a surgical antiseptic used previously by pro-Kremlin actors to douse on opposition politicians, including Navalny.

Russia's FSB security service also announced arrests of Russians it said were aiding Ukrainian forces or planning to carry out sabotage at military and transport facilities.

Ukrainian attacks have extended well beyond border regions, with Kyiv's forces targeting oil facilities deep inside Russian territory.

The governor of the Samara region – around 500 miles from the front lines – said Saturday that Ukrainian drones had targeted two oil refineries, igniting a blaze at one of them.

A defense source in Kyiv told AFP the attack was planned by the SBU security services as part of "a strategy to disrupt the economic potential of Russia."

"Each such defeat reduces the flow of petrodollars that feeds Russia's war economy," the source said.