Russians vote in nationwide parliamentary poll
There have been allegations of violations of election law, with Russia's only independent monitoring group, Golos, logging 5,300 complaints, BBC News reports.
Its head, Lilya Shibanova, was held at a Moscow airport after refusing to hand over her laptop.
Mr Putin has accused foreign countries of meddling in election preparations.
The Golos monitors, who are non-party political, are funded largely by the US and EU.
Russian MPs have questioned why the foreign-funded organisation - whose name means "voice" or "vote" - is allowed to monitor Russian elections.
Long voting day
Voting for representation in the country's 450-seat lower chamber takes place across Russia's nine time zones with polls closing at 1700GMT on Sunday in Kaliningrad.
As people in European Russia were going to bed, polls were opening in the far east.
Voting began in Moscow some hours after the Russian far east
In Vladivostok, sailors from Russia's Pacific fleet were among those queuing to vote.
Warrant Officer Nikolai Ponomaryov said he was voting for United Russia, because the party was supporting the armed forces.
In the far north, above the Arctic circle, voters braved temperatures of -26C to cast their ballots.
Polling stations have now opened in the capital, Moscow.
'Provocation'
On Friday, Golos was fined the equivalent of $1,000 (£641) by a Moscow court for violating a law that prohibits publication of election opinion research for five days before a vote.
Many of the allegations about electoral malpractice involved the politically dominant United Russia, chaired by Prime Minister Vladimir Putin.
Ms Shibanova said officials had taken her laptop on the pretext that it had illegal software.
"This is a provocation directed personally against me," she said, alleging that the authorities were trying to prevent her from travelling to the European parliament next week.
Seven parties have been allowed to field candidates for parliament this year - down from 11 in 2007.
The outgoing parliament, or State Duma, is dominated by Mr Putin's party, with seats also held by the Communist Party, the nationalist Liberal Democrats and the social-democratic Fair Russia.
A presidential poll will be held on 4 March, when Mr Putin will stand for election having served two previous terms in the post.
In a televised address on Friday, President Dmitry Medvedev insisted Russia's political parties enjoyed "free and equal competition" ahead of the election.
Without naming United Russia, he urged voters to choose "responsible politicians, who can help improve our people's living standards in practice, and who will be guided in their actions by the interests of voters and national interests".
To learn more go to www.bbc.co.uk
