Opposition figurehead Alexei Navalny said on Tuesday his e-mail and Twitter account have been hacked into.
"It was apparently [done] through the computers and iPads seized during a search," Navalny wrote on his LiveJournal blog in the early hours of Tuesday morning.
"This is the Investigative Committee for you," he said.
Police raided the apartments of a dozen opposition leaders ahead of a mass protest against President Vladimir Putin earlier this month.
A number of obscene entries appeared on Navalny’s Twitter overnight.
The FBI-style body dismissed Navalny's allegations as "untrue."
"The spreading by A[lexei] Navalny of false information is seen by the Investigative Committee as pressure, and attempts to discredit the investigation," it said in a press release.
Navalny, the founder of the anti-graft website Rospil and shareholder rights campaigner, is one of several activists being investigated in relation to clashes between protesters and police on eve of President Putin's May 7 return to the Kremlin for a third term. He spent 15 days in jail last month on protest-related charges.
Navalny was also ordered to report to officials for questioning on Tuesday over two rallies in December, Navalny's spokeswoman, Anna Veduta, wrote on her Twitter account.
On Monday, Navalny was elected to the board of the state-controlled airline Aeroflot.
