MOSCOW (Reuters) - Russian investigators opened a criminal case into the theft of a picture found in the home of a prominent critic of President Vladimir Putin on Friday, in what his lawyer described as part of a Kremlin campaign against him.

The case is one of several involving Alexei Navalny, an anti-corruption blogger under house arrest in connection with a fraud case after helping lead a wave of mass street protests against Putin in 2011-2012.

He is already serving a five-year suspended sentence on a separate 2013 theft charge.

Investigators said on Friday that the picture seized in a search of Navalny's home had been stolen from its artist, who had hung it outside on central street in the city of Vladimir, some 170 kilometers northeast of Moscow.

"An unidentified person secretly stole the painting ... which had been hung for all to see on one of the central streets of the city of Vladimir," the Investigative Committee said in a statement.

"This picture belongs to one of the local artists. Its theft has brought significant harm to the author," it said, without naming the artist.

Navalny's lawyer said investigators had searched his client's apartment overnight, arriving at four o'clock in the morning. "The search is yet another attempt to put pressure on him," his lawyer Vadim Kobzev said.

Navalny won a rare judicial reprieve in April when a Moscow court sent back another theft case against him to prosecutors for overhaul, delaying his trial.

He could be jailed for years if found guilty on charges of stealing more than 30 million rubles ($832,700) from two companies, one of them an affiliate of French cosmetics firm Yves Rocher. He and his brother Oleg, who is also charged, deny any guilt.

(Writing by Alissa de Carbonnel; Editing by Tom Heneghan)