On Tuesday and Wednesday, several Internet forums carried reports of radioactive emissions from the Leningrad Nuclear Power Plant near St. Petersburg, and of a planned evacuation of local residents.
A spokesman for the Rosatom state nuclear corporation said the cyber attacks had been planned and coincided with the release of the reports.
"People who stand to lose out from the Russian nuclear power industry's development have an incentive to spread false rumors of an accident at the nuclear plant," he said.
"This was a planned action by hackers, which has brought down almost all sites providing access to the Automatic Radiation Environment Control System (ASKRO), including the Leningrad NPP site, the rosatom.ru site, and others. For several hours users were unable to reach the sites and obtain reliable information on the situation at the plant."
ASKRO is part of a permanent environment and sanitary control system, one of whose functions is to inform the population on radiation security. Access to the system is open to all visitors on a number of Russian nuclear industry websites. The system works in real-time.
Access to ASKRO data has now been fully restored, the spokesman said.
He said this was not the first incident of its kind in Russia. Last year, after similar false reports of an accident at the Volgodonsk nuclear plant, several dozen people, believing they could offset radiation damage by consuming large amounts of iodine, fell ill after poisoning themselves.