Polls show that Putin, who has dominated Russia's political landscape for the last 17 years as either president or prime minister, is on course to comfortably win another six-year term. That would allow him to rule until 2024, when he'll turn 72.
The former KGB officer is running as an independent, a move seen as a way of strengthening his image as a "father of the nation" rather than as a party political figure.
The ruling United Russia party and the Just Russia party have both said they will support him.
Allies laud Putin for restoring national pride and expanding Moscow's global clout with interventions in Syria and Ukraine.
But opposition leader Alexei Navalny, who has been barred from the election over a suspended prison sentence he says was fabricated, says Putin has been in power too long and that his support is artificially maintained by a biased state media and an unfair system which excludes genuine opponents.
Navalny has called for a boycott of the election, raising the prospect of large-scale protests and clashes with the police.
(Reporting by Katya Golubkova and Polina Nikolskaya; Writing by Andrew Osborn; Editing by Andrew Bolton)
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