Russians were voting on Sunday in an election set to hand President Vladimir Putin a fourth Kremlin term but slammed by the opposition as a sham as tensions with Britain escalated over the poisoning of a former Russian double agent.

With the vast country stretching across 11 time zones, polls opened in the Russian far east at 2000 GMT on Saturday and will close in Kaliningrad, the country's exclave on the EU border, at 1800 GMT on Sunday.

Putin's main challenger Alexei Navalny has been barred from taking part in the poll for legal reasons, and the result of the election is in little doubt, with overall turnout likely to provide the only element of surprise.

A total of 107 million Russians are eligible to cast their votes in Sunday's election in the world's biggest country, but some analysts say that after 18 years of leadership — both as president and prime minister — Putin fatigue may be spreading, with many Russians are expected to skip the polls.

The Kremlin needs a high turnout to give greater legitimacy to a new mandate for Putin, who is already Russia's longest-serving leader since Joseph Stalin.

By 0700 GMT, turnout stood at 16.55 percent, compared with 6.53 percent at the same hour during the 2012 vote, said Central Electoral Commission head Ella Pamfilova.

Many of those who cast their ballots voted for Putin, praising him for lifting the country out of the post-Soviet quagmire.

"Of course I'm for Putin, he's a leader," said Olga Matyunina, a 65-year-old retired economist.

"After he brought Crimea back, he became a hero to me. Last election I didn't vote for Putin, I don't even remember who I voted for."

At many polling stations the atmosphere was festive, with patriotic songs blasting out of speakers outside and cheap food available to voters.

'Six years of slavery'

Casting his ballot in Moscow, Putin said he would be pleased with "any" result that gave him the right to continue serving as president.

"I am sure the programme I am offering is the right one," he said.

Navalny, 41, has denounced the election as a sham and urged Russians to boycott the vote.

He has deployed more than 30,000 observers to monitor the polls and on Sunday, his team — which calls the vote "a staged procedure to re-appoint Putin" — began publishing a rolling list of violations from polling stations.

"Those who said that 'there would be fewer falsifications during these elections because Putin has already won over everyone' have made a mistake," he said.

Navalny, who faces a 30-day jail sentence, had been expected to spend election day behind bars but remained free on Sunday.

"I will not go to vote. What for?" said Boris Limarev, a 39-year-old manager, as he walked his dog near a polling station in southwest Saint Petersburg. "It's clear to everyone who will be elected."

"And the rest of the candidates are clowns," interjected his wife Anna, 35.

"Another six years of slavery," said a piece of paper made up to look like a ballot which was attached to a porch on a central Moscow street — in an apparent reference to Putin's next term in office.

Geopolitical tensions

Ahead of the poll, a new crisis broke out with the West as Britain implicated Putin in the poisoning of former double agent Sergei Skripal with a Soviet-designed nerve agent.

In response, London expelled 23 Russian diplomats, prompting Moscow to respond in kind. Also this week, Washington hit Russia with sanctions for trying to influence the 2016 US election.

Since first being elected president in 2000, Putin has stamped his total authority on Russia muzzling opposition, putting television under state control and reasserting Moscow's posture abroad.

He has sought to use the campaign to emphasise Russia's role as a major world power, boasting of its "invincible" new nuclear weapons in a key pre-election speech.

His previous Kremlin term has been marked by a crackdown on the opposition after huge protests, the annexation of Crimea, support for an insurgency in east Ukraine, a military intervention in Syria and the introduction of Western sanctions that contributed to a fall in living standards.

But the 65-year-old former KGB officer is certain to extend his term to 2024 despite a litany of domestic problems such as widespread poverty and poor healthcare following a lacklustre campaign.

'Economy is terrible'
In Saint Petersburg, Antonina Kurchatova said she voted for Putin but was just hoping things would not get worse.

"I very much like his foreign policy. He's doing everything right. But as far as the economy is concerned, everything is terrible," the 40-year-old told AFP.

Putin: Villain abroad, hero at home

State-run pollsters predict Putin will take just under 70 percent of the vote, with the independent Levada Centre — branded a "foreign agent" — barred from releasing any research related to the election.

Sunday marks four years since Putin signed a treaty declaring Crimea to be a part of Russia in a move that triggered the outbreak of a pro-Kremlin insurgency in east Ukraine, in a conflict that has claimed more than 10,000 lives.

Kiev has said Russians living in Ukraine would not be able to vote as access to Moscow's diplomatic missions would be blocked.

Anything but predictable
Putin is standing against a motley crew of seven Kremlin-approved challengers, including millionaire communist Pavel Grudinin and former reality TV host Ksenia Sobchak, but none are polling more than eight percent.

Overall turnout is expected to be between 63 and 67 percent, according to state-controlled pollsters.

Authorities have pulled out all the stops to ensure a huge turnout, offering food discount vouchers and prizes for the best selfies taken at polling stations after a sexually charged online campaign.

While the result of the vote was a foregone conclusion, the next six years would be anything but predictable, said Tatyana Stanovaya, an analyst with Moscow's Center for Political Technologies.

"No one can tell now just how emotional and radical Putin's actions will be under increasing geopolitical pressure," she told AFP, adding the role of the military and the security service would increase.

"The regime is inclined to become more closed and tough."