MOSCOW (Reuters) - Russian investigators said on Wednesday a prominent critic of President Vladimir Putin had gained his qualifications as a lawyer illegally, piling more pressure on the opposition leader who already faces three criminal investigations.

Alexei Navalny, an anti-corruption blogger, was a leader of a street protest movement against Putin's 13-year rule that started after mass allegations of fraud in a parliamentary election in December 2011.

But the protests have withered and made no significant inroads into the president's grip on power.

Russia's federal Investigative Committee, which answers only to Putin, has launched criminal charges against several leaders of the street protests. Putin's critics accuse the president of cracking down on dissent since his return to the Kremlin last May.

The committee said on Wednesday that Navalny had in the past provided fraudulent paperwork to confirm the work experience he had needed to become qualified as a lawyer.

"An investigation revealed the fact that Alexei Navalny illegally obtained his lawyer's status," said spokesman Vladimir Markin.

Navalny, who worked as a commercial lawyer before taking up political activism, said there were no grounds to strip him of his legal credentials.

The 36-year-old has already been charged with stealing timber worth more than $500,000 from a state company in 2009; embezzling up to $3.24 million from a political party in 2007; and, along with his brother, cheating a mail-transport company out of $1.79 million.

He has denied any wrongdoing and dismissed the charges against him as politically motivated.

Navalny and other opposition figures say Putin has used the justice system to persecute his adversaries and the parliament to adopt laws aimed at stifling the opposition movement.

(Reporting by Gabriela Baczynska; Editing by Pravin Char)