(Reuters) – President Vladimir Putin will seek to tighten his grip on power on Sunday in a Russian election that is certain to deliver him a landslide victory though opponents called on people to stage a symbolic protest against his rule at polling stations.

Putin, who rose to power in 1999, is poised to win a new six-year term that, if he completes it, would make him Russia's longest-serving leader for more than 200 years

The election comes just over two years since Putin triggered the deadliest European conflict since World War Two by ordering the invasion of Ukraine in what he casts as a defensive "special military operation".

War hangs over the three days of voting that ends on Sunday: Ukraine over recent days repeatedly attacked oil refiners in Russia with drones, shelled Russian regions and sought to pierce Russian borders with proxy forces - a move Putin said would not be left unpunished.

Putin, 71, has warned the West that any meddling in the vote will be considered an act of aggression. While his re-election is not in doubt given a lack of any significant rival candidate he wants to show that he has the overwhelming support of Russians.

The Kremlin has sought a high turnout and as polls opened for a third day in western Russia, officials said the turnout in the first two days had already reached 60% nationwide. An exit poll will be published shortly after voting ends at 1800 GMT.

Supporters of Alexei Navalny, who died in unexplained circumstances at an Arctic prison last month, have called on Russians to come out at a "Noon against Putin" protest to show their dissent against a leader they cast as a corrupt autocrat.

"Today we want to say to all of us - noon is the very beginning," the "Noon Against Putin" initiative wrote on their Telegram early Sunday.

"Yes, some of us are scared. Yes, the choice is not easy. But we are the people. And we will cope with both the choice and the responsibility."

Over the previous two days, there were scattered incidents of protest as some Russians set fire to voting booths and poured dye into ballot boxes, drawing a rebuke from Russian officials who said they were scumbags and traitors.

About 100 people came out at one polling station at noon (0500 GMT) in the Siberian city of Novosibirsk. Abroad, at Russian voting stations from Thailand to Sydney and from Bishkek to Tashkent queues formed at noon, though it was not immediately clear if the Russians were protesting or simply voting.

Many Russians opposed to Putin have left Russia - with many young men fleeing abroad in 2022 after the start of the war in Ukraine and a mobilisation order.

The West, variously, casts Putin as an autocrat, a war criminal and a killer. U.S. President Joe Biden last month dubbed him a “crazy SOB” and U.S. officials say he has enslaved Russia in a corrupt dictatorship that leads to strategic ruin.