Russian observers have warned they are seeing widespread violations in elections expected to return Vladimir Putin to the Kremlin, despite efforts to give the appearance of a fair vote.
Mr Putin, who was president in 2000-2008, is all but certain to easily defeat his four challengers.
But if credible evidence of vote manipulation emerges, it would call into question the legitimacy of his win and bolster the determination of opposition forces to continue the unprecedented wave of protests that arose in December.
The independent elections watchdog agency Golos said it was receiving reports of so-called "carousel voting", in which bus loads of voters are driven around to cast ballots multiple times.
"There have been many people voting more than once, driven around in buses in large numbers" in Moscow, said Golos head Lilia Shibanova, who added similar reports had been received from Novosibirsk, Russia's third-largest city, and the city of Barnaul in southern Siberia.
Alexei Navalny, one of the opposition's most charismatic leaders, said observers trained by his organisation also reported seeing extensive use of carousel voting.
"These are not going to be honest elections, but we must not relent," said Mikhail Gorbachev, the last leader of the Soviet Union who has grown increasingly critical of Mr Putin, as he cast his ballot. "Honest elections should be our constant motto for years to come."
Evidence of widespread vote fraud in a December parliamentary election set off the protests against Mr Putin, who has remained Russia's paramount leader after moving into the prime minister's office four years ago because of term limits.
They were the largest public show of anger in post-Soviet Russia and demonstrated growing frustration with corruption and political ossification under Mr Putin.
Some polling stations in Moscow that had been instructed to rig the vote in December were told to make sure the election was held "in full accordance with the law", an election official said. He said the instructions were handed down at a meeting attended by the heads of about 50 polling stations.