Earlier in the year, Russia enacted a law that gave the government powers to block websites without explanation.
In March, Moscow blocked the blog of Mr Navalny, along with two news sites and a organisation run by Garry Kasparov - a vocal critic of the Russian government.
In a statement, Russia's prosecutor general's office said the blocks were imposed because of the sites' role in helping stage illegal protests.
Earlier this week, Twitter blocked access to an anti-Kremlin account that often publishes leaked government documents, following a request by Russia's federal communications agency Roskomnadzor.
