The head of Russia's second-largest political party is alleging widespread violations in the election for a new parliament in which his party is widely expected to gain seats.
Communist Party chief Gennady Zyuganov said on Saturday - the second of three days of voting in the election - that police and the elections commission must respond to reports of "a number of absolutely egregious facts" including ballot-stuffing in several regions.
The Golos election-monitoring movement and independent media also reported violations including vote-buying and lax measures for guarding ballots at polling stations.
Central Elections Commission head Ella Pamfilova said later on Saturday that more than 6200 ballots have been annulled in five regions for procedural violations and ballot-stuffing.
The United Russia party, which is diligently loyal to President Vladimir Putin, appears certain to retain its dominance in the State Duma, the lower house of parliament.
Still, some projections suggest the party could lose its current two-thirds majority which is enough to change the constitution.
The Communists are expected to pick up the biggest share of any seats lost by United Russia.
Although the Communists generally support Kremlin initiatives in the parliament, their gaining seats would be a loss of face for United Russia.
The Communists are seen as potentially benefiting from the "Smart Voting" program promoted by imprisoned opposition leader Alexei Navalny and his team, which aims to undermine United Russia by advising voters on which candidates are in the strongest position to defeat United Russia's candidates.
However, it's unclear how effective the program will be after Apple and Google removed Smart Voting apps from their stores under Kremlin pressure.
Authorities previously blocked access to its website.
Navalny's organisations have been declared extremist, blocking anyone associated with them from running for office, thereby eliminating most significant opposition candidates from the election.
In St Petersburg, voter Pavel Ivanov said he had access to the service and followed its advice to vote for a small party that "does not meet my preferences to the full extent but (will) present a certain opposition to the ruling party".
Zyuganov said the party has tallied at least 44 incidents of voting violations and the Communists have applied for permits to hold protests next week after the voting ends on Sunday.
On Saturday, the news website Znak said a resident of the Moscow region was offering 1000 roubles ($A21) to people who voted for United Russia.
The publication said it called the man, who said the payment would come if the caller provided evidence of their vote through a messaging app.
The Golos movement cited reports from its observers and local news media of an array of apparent violations, including ballots being stored overnight in a cabinet with a broken door and of envelopes for storing ballot tallies appearing to have been opened and then resealed.
On the first day of voting on Friday, unexpectedly long lines formed at some polling places and independent media suggested this could show that state institutions and companies were forcing employees to vote.
But despite those lines, overall turnout appeared to be desultory.
Pamfilova, the elections commission head, said about 25 per cent of the electorate had cast ballots by 3pm on Saturday, about halfway through the voting.
Australian Associated Press