Popular Russian radio station Moscow Echo and election monitoring group Golos said their websites were the victims of massive cyber 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 "distributed denial of service" (DDoS) 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 Sunday's polls but with a reduced majority, has denounced non-governmental organisations like Golos, comparing them to the disciple Judas who betrayed Jesus.