Popular Russian radio station Moscow Echo and election monitoring group Golos said their websites were the victims of massive distributed denial of service (DDoS) attacks, while several opposition news sites were inaccessible. "The attack on the website on election day is clearly an attempt to inhibit publication of information about violations," Moscow Echo editor-in-chief Alexei Venediktov wrote on Twitter.
Golos said it was the victim of a similar attack while several other opposition news sites were down. The Moscow Echo is popular among the liberal opposition although it is owned by state gas giant Gazprom.
Prime Minister Vladimir Putin, whose United Russia party is expected to win the polls but with a reduced majority, has denounced non-governmental organisations like Golos, comparing them to the disciple Judas who betrayed Jesus.
Russia has seen an upsurge in Internet penetration since the last elections in 2007 and analysts have said the explosion of critical material on the web poses one of the biggest challenges to United Russia's grip on power.
Golos said on Twitter that its main website as well as the website 'Map of Violations' detailing claims of fraud across Russia were under "massive DDoS attacks". Golos head Liliya Shibanova said that the authorities seemed especially angry at their Map of Violations project, where people could upload any information or evidence of election violations.
"It's a very expensive operation," Shibanova said of the attacks. "It's a big organisation with plenty of means that must have done it."
Its entire operation appeared to be under siege Sunday, with spokeswoman Olga Novosad telling AFP: "Our email is not working, and we only have Skype to communicate with our regional network."
Faced with such odds, Golos is relying on blogs and Twitter to record violations in Sunday elections, she said. Moscow Echo has also decentralised its website and broadcast streaming to other Internet sources.
"Everyone knew that the monsters would do anything to rig (elections)," wrote Moscow Echo presenter Matvey Ganapolsky on Twitter. "That's okay, we're patient." He vowed to publish all the incoming news on his blog.
The website of oppositional weekly New Times, known to publish investigative reports about government officials and feature columns by jailed oil tycoon Mikhail Khodorkovsky, was also down, along with news website slon.ru.
Both have extensively covered the elections campaign and published opinion pieces on voting tactics in what opposition commentators have called a sham election with pre-determined results.
Bolshoi Gorod, a Moscow weekly magazine popular with the hipster youth, was also attacked at noon on Sunday. "We are in good company with Slon.ru, Moscow Echo, and Map of Violations," editor Filipp Dzyadko wrote on Facebook.
Popular business daily Kommersant was not working for the fourth consecutive day after it was hacked on Thursday. Hackers switched its IP address.
Russian bloggers also complained of their inability to access their accounts on popular blogging platform Livejournal.com. The website has been a victim of repeated DDoS attacks throughout the week and worked intermittently. "Yes, they are still attacking. They must have a mountain of money," head of Live Journal Ilya Dronov wrote on his Twitter blog Saturday.