"This is the Investigative Committee for you," Navalny wrote on his LiveJournal blog, adding that the hacking was apparently done though computers and iPads that were seized during a search, the Russian news agency RIA Novosti reported.
Earlier this month, Russian police raided the apartments of a dozen opposition leaders ahead of a planned protest against President Vladimir Putin.
Navalny is one of several activists being investigated in connection with clashes between protesters and police on the eve of Putin's election to a third term May 7.
He spent 15 days in jail last month on protest-related charges.
The Investigative Committee has denied that it was responsible for the hacking, ITAR-Tass reported.
A committee official said the panel would investigate the matter if Navalny files a formal complaint.
Navalny is the founder of the anti-graft Web site Rospil.