Putin vows revenge for Ukrainian attacks as Russians vote

President Vladimir Putin on Friday vowed a strong military response to a string of Ukrainian attacks on Russia's border that he described as an attempt by Kyiv to derail his bid for re-election.

Putin addressed his security council on the first day of the three-day vote that is also being held in occupied territories of Ukraine and with no opposition candidates allowed to contest the ballot.

He promised a harsh response to waves of fatal Ukrainian aerial attacks on the frontier regions of Belgorod and Kursk that have also seen fierce fighting in recent days with pro-Kyiv sabotage groups.

"These strikes by the enemy do not and will not go unpunished," the long-time Russian leader said in comments aired on state-run television.

"This is an attempt to interfere with the presidential election," the 71-year-old Russian leader added.

Putin has been in power in Russia since the last day of 1999 and is set to extend his grip on power until 2030.

The Kremlin distributed images showing Putin voting online at his office computer and waving to the camera after issuing the vow to strike back against Ukraine, and as Russian strikes killed at least 19 in the Ukrainian port city of Odesa.

He is running unchallenged, having barred two candidates who opposed the conflict in Ukraine and around one month after his main opponent, Alexei Navalny, died in an Arctic prison in unexplained circumstances.

Authorities have encouraged Russians to head to the polls out of patriotic duty, with the Kremlin saying the election will prove the country is fully behind Putin's Ukraine assault.

AFP journalists at polling stations in the capital interviewed Russians supportive of Putin but also heard from voters who said they were pressured into participating.

"If I did not come to the elections, I would have had problems," Nadezhda, a 23-year-old ballerina, told AFP.

"Most young people understand anyway that they can't do anything, that they can't change anything," said Nadezhda, who has spent her life living under Putin's rule.

The first day of voting was marred by acts of vandalism in polling stations, with at least nine arrests for pouring dye into ballot boxes and arson attacks.

In Moscow, video showed a woman setting a voting booth alight, filling a polling station with smoke, while another showed a woman pouring green dye into a ballot box.

Four other people in the Russian regions of Voronezh, Karachay-Cherkessia and Rostov were detained for similar offences. In Saint Petersburg and in Siberia, women were detained for throwing Molotov cocktails at polling stations.

A man was also detained for lighting fireworks inside a polling station in the Urals city of Chelyabinsk.

It was unclear whether the spate of polling station incidents was a coordinated protest against the ballot or isolated incidents.

"It seems Russians have chosen their form of protest: ruining voting booths," said Tatiana Stanovaya, a respected Russian political analyst.

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

Many residents who came to vote for Putin in Moscow were motivated by one concern: hopes that Russia would be victorious in Ukraine.

Lyudmila, a 70-year-old who was one of the first to vote in a central Moscow school, backed Putin and was hoping for "above all, victory".

Voting was also being organised in five occupied territories of eastern Ukraine that Russia claims to have annexed, including the Crimean peninsula.

In the occupied city of Skladovsk in southern Ukraine, Russian-installed officials said an explosive device was detonated at a voting site but there were no casualties.

Armed soldiers in full combat gear accompanied election officials in the eastern Donetsk region as they set up mobile voting stations on small tables in the street or on the hoods of Soviet-era cars.

Kyiv said holding the election in eastern Ukraine and Crimea, which Russia annexed in 2014, was "illegal". The move was also denounced by UN Secretary-General Antonio Guterres and more than 50 member states.

Russian deputy UN ambassador Dmitry Polyanskiy said he would not comment on criticisms regarding "the domestic affairs of our country".

Russia's opposition groups have called for voters to form queues at polling stations on Sunday, the final day of voting, as a form of protest.

Moscow prosecutors warned that they would punish anybody involved in mass rallies.