Two liberal Russian media outlets and an election watchdog said their sites had been shut down by hackers intent on silencing them over allegations of violations in a parliamentary vote on Sunday.
One publication accused state bodies of interference.
Prime Minister Vladimir Putin's ruling United Russia party could see its vast parliamentary majority cut back in the vote, which is widely viewed as a test of his popularity ahead of an expected return to the presidency early next year.
Sites belonging to the Ekho Moskvy radio station, online news portal Slon.ru and the Western-financed watchdog Golos went down at around 8 a.m. (0400 GMT).
"There is the feeling that the Central Election Commission, the prosecutors and the hackers are acting together," Maxim Kashulinsky, general director of Slon.ru, told Reuters.
President Dmitry Medvedev, who is stepping aside so that Putin can return to the presidency next year, has dismissed talk of electoral fraud. Neither the general prosecutor's office nor the Central Election Commission could be reached for comment.
Kashulinsky said the news portal was targeted because it supported a Golos project which has an interactive map displaying reported campaign violations across the country ahead of the vote in which Russians choose a new parliament.
Opposition parties said on Sunday the election was unfair and accused authorities of violations across the country, from ballot stuffing to pressuring voters at polling stations to choose United Russia.
Political analysts say the centralisation of power under Putin during his eight-year presidency until 2008 encourages abuses because many regions compete to secure the highest vote for United Russia -- a show of loyalty they hope will be rewarded by a bigger share of state handouts.
Ekho Moskvy's editor-in-chief Alexei Venediktov wrote on his Twitter account: "It is obvious that the election day attack on the site is part of an attempt to prevent publishing information about violations."
The director of Golos, which said its site was also forced down in a cyber attack, was detained for 12 hours in a Moscow airport when she arrived on Saturday. Liliya Shibanova's laptop was also seized in what the group said was a blatant attempt to stop it monitoring the election.
Last week Moscow prosecutors launched an investigation into Golos after lawmakers objected to its Western financing.
Several other independent media sites, including prominent opposition magazine The New Times, also suffered shutdowns on Sunday, but the cause was not immediately clear.
The online radio belonging to business daily Kommersant -- which had asked listeners to post any voting violations they had witnessed -- was also down on Sunday.
Though he is by far Russia's most popular politician, opinion polls suggest some Russians have grown wary of Putin and his overarching domination of national politics.
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