Russians have opened the door to Vladimir Putin staying in power until 2036 by voting overwhelmingly for constitutional changes that will allow him to run again for president twice, but critics said the outcome was falsified on an industrial scale.

Official results show that the former KGB officer who has ruled Russia for more than two decades as president or prime minister had easily won the right to run for two more six-year terms after the current one ends in 2024.

That means the 67-year-old could rule until the age of 83.

The Central Election Commission said final results of the week-long vote show that almost 78% of the electorate had backed the changes.

The head of the commission said the ballot had been transparent and that officials had done everything to ensure its integrity.

"Thank you very much for your support and trust," Mr Putin said in a televised address to the Russian people.

He claimed the series of amendments were necessary to "improve the political system and fortify social guarantees."

But an independent monitoring group said the vote was deeply flawed.

Opposition politician Alexei Navalny described the vote as an illegitimate and illegal show designed to legalise Mr Putin's presidency for life.

"We'll never recognise this result," Mr Navalny told supporters in a video.

He said the opposition would not protest for now because of the coronavirus pandemic, but would do so in big numbers in the autumn if its candidates were blocked from taking part in regional elections or their results were falsified.

"What Putin fears most is the street," Mr Navalny added. "He ... will not leave until we start to take to the streets in the hundreds of thousands and in the millions."

"We need to remind the authorities that we exist and that there are tens of millions of us who do not want Putin to rule until 2036," Andrei Pivovarov, an activist who had campaigned for a 'no' vote, said.

Vladimir Putin arriving to cast his vote yesterday at a polling station in Moscow

Russians had been encouraged to back Mr Putin's power move, described by critics as a constitutional coup, with prize draws offering flats and an ad campaign highlighting other constitutional amendments in the same reform bundle, such as pensions protection and a de facto ban on same-sex marriage.

One-off payments of 10,000 roubles (€126) were transferred to those with children at Mr Putin's order as people headed to polling stations yesterday, the last day of the vote, held over seven days to try to limit the spread of the virus.

Turnout was 65%, election officials said.

Edward Lucas, senior vice president at the Centre for European Policy Analysis (CEPA) and author of a 2008 book on Mr Putin's Russia, 'The New Cold War', said he believed the president was in a race against time to push the vote through.

"Putin wants to get the constitutional change nailed down now because he's worried about what he's going to be facing in the coming years and the pandemic and the economic and social stress that that brings, just accentuates his existing problems," Mr Lucas told RTÉ News.

He said there is no doubt that this election was "skewed both in terms of the inability to campaign against the constitutional changes, the way in which the changes were presented, and the way in which the votes were counted- so its a kind of triple skew."

Mr Lucas added: "You can usually assume that the authorities are scared of the something. If they thought this was genuinely popular they would allow real elections and expect to win it."

Mr Lucas said that many Russians now feel apathetic.

"They don't take formal politics seriously at all. They know the whole system’s rigged and they just rub along with it the best they can. And even Mr Putin, for all his powers cannot summon up enthusiasm, he can only summon up compliance."

He added that the real problem the president faces is that the regime does not have a mechanism by which power can successfully be transferred to another leader.

Mr Lucas said he believed that Mr Putin could also be buying more time to search for a successor that he trusts, concluding "the best way to make sure you don’t end up in jail is to make sure you stay in power."

Mr Putin, already the longest-serving leader in modern Russian history since Soviet dictator Josef Stalin, made no mention of how the changes could affect his own career in an eve-of-vote speech on Tuesday.

He has said he has yet to decide on his future. Critics say they are sure he will run again, but some analysts say he may want to keep his options open to avoid becoming a lame duck.

At 60%, according to the Levada pollster, his approval rating remains high but well down on its peak of nearly 90%.

With Russia reporting thousands of new Covid-19 cases each day, opponents have been unable to stage protests but have mocked the vote online, sharing photographs of polling stations in apartment stairwells, supermarket trolleys and the boot of a car.

Additional reporting Eleanor Burnhill