Navalny, who is held at the maximum security IK-6 penal colony at Melekhovo, about 250 km (115 miles) east of Moscow, where he has endured frequent spells in solitary confinement, said an unspecified "eyewitness" had described Prigozhin's visit to him.

He said that Prigozhin had offered convicts a pardon if they survived six months with Wagner, and that between 80 and 90 of them accepted his offer after being given five minutes to consider it. He did not say when the alleged visit took place.

Prigozhin, in comments released by his press service, declined to confirm the visit but said he saw nothing wrong in offering prisoners a "second chance" in life.

"No one can deprive a person of the right to defend his motherland, his mother and his family by all available means," he said.

In September, footage was released of Prigozhin, who served nine years in prison during the Soviet era for crimes including theft and fraud, offering a similar deal to convicts at a prison in the city of Yoshkar-Ola, 760 km (470 miles) east of Moscow.

The Wagner Group, which Prigozhin has said he founded in 2014, has become increasingly prominent during the Ukraine war, including during a brutal, long-running battle for the small city of Bakhmut.

Navalny, President Vladimir Putin's most outspoken domestic critic and a trenchant opponent of the war in Ukraine, is currently serving sentences totalling 11-1/2 years on charges including fraud and contempt of court. Navalny and Western governments say the charges against him are politically motivated.

In his Twitter post, he said the recruitment of convicts to fight in Ukraine showed the corrosion of the Russian state.

"The foundations of law are being destroyed in Russia for the sake of a future defeat in a shameful war," said Navalny, who is able to post on social media through his lawyers and allies.

In his response, Prigozhin took issue with the fact that many of Navalny's supporters had been allowed by the state to flee abroad.

"They should have been gathered, formed into one penal battalion and handed over to me," he said. "And have no doubt, they would all have died heroes."

(Reporting by Reuters; Editing by Alex Richardson)