Russia's anti-corruption blogger Alexey Navalny has had his email and Twitter accounts breached by a hacker.
Navalny, who was at the forefront of December's historic mass anti-government rallies, was arrested during protests that followed Putin's 7 May inauguration as Russian president. He was released after two weeks.
He is also a shareholder activist who campaigned for better corporate governance at state firms.
His nomination to the board of Aeroflot by Alexander Lebedev, owner of the Independent, puts him in an influential position in Russia.
"Alexey has made his name as a shareholder activist, promoting and fighting for information and transparency at Russian state-controlled companies," said associate and investment manager Vladimir Ashurkov, who manages a fund set up by Navalny to fight corruption.
But in an interview with Izvestia, a pro-Putin newspaper, "Hacker Hell" claimed that he broke into Navalny's online accounts and stole 17,000 emails which he plans to publish.
Hacker hedll claimed Navalny "is a thief, a crook and an informer, and on top of that, he lies constantly."
It is not the first time Hacker Hell has attacked Navalny. In October he claimed to have hacked into Navalny's email account.
Navalny's apartment and office have been repeatedly raided by police.
Navalny has accused the authorities, in the form of the Investigative Committee, of leaking his passwords on other occasions. Compromising email and social network accounts is common in Russia but no hacker has ever been arrested for cyber-crimes.
Navalny asked investigators to look into whether police had helped hackers breach his email and Twitter accounts.
"My e-mail has been broken into, and through that, my Twitter," Navalny wrote on his LiveJournal blog. "It's obvious it was [hacked] from the computers and iPads seized during the search."
The blogger alleged that investigators might have shared passwords acquired during searches of his house with hackers in a bid to gather evidence for a suit against him.
The hacker also changed the account's avatar to a photograph of Alexandre Dumas' hero D'Artagnan from the Three Musketeers and the caption "You are all paedophiles and I am D'Artagnan" and a new profile description "Crook and thief Alexei Navalny 2.0".
The Investigative Committee denied any involvement in the Navalny hack and accused him of attempting to discredit police investigations into him, according to the Moscow Times.