By Andrew Osborn

MOSCOW (Reuters) - A Russian opposition activist who became the first person to be jailed under a new law for repeatedly staging peaceful anti-Kremlin protests says he is being tortured in prison and fears for his life.

A Moscow court sentenced Ildar Dadin, 34, to three years in prison in December for holding a series of one-man protests, although his term was reduced to two and a half years on appeal.

Russian rights activists regard Dadin, an outspoken critic of President Vladimir Putin and his policies, as a political prisoner. The authorities say Dadin broke a law, introduced after big anti-Kremlin protests, that criminalizes anyone who violates protest rules more than twice in 180 days.

In a letter to his wife from prison in northwest Russia, published on Tuesday by the online news portal Meduza, Dadin said he was being subjected to beatings in which around 10 prison guards would kick him at the same time.

He said he had also had been hung up like a piece of meat with his hands handcuffed behind his back, that he had been stripped and threatened with rape, and that the prison governor had warned him he would be murdered if he complained.

"Regular beatings, bullying, humiliation, insults, intolerable detention conditions – it's happening to the other prisoners as well," the letter said.

Dadin said if he was subjected similar treatment again, "it's unlikely that I'll last more than a week".

Anastasia Zotova, his wife, said on social media she had felt something was wrong when prison authorities had not allowed her to speak to her husband on the phone or to visit.

Valery Maksimenko, the deputy head of the prison service, told the Interfax news agency that a medical investigation had not found any injuries on Dadin, and that Dadin had confirmed as much in a video interview.

He said an investigation into the allegations was under way. A spokeswoman for the prison service did not respond to a request to comment on whether Dadin had retracted his claims.

Dmitry Peskov, a spokesman for Putin, told reporters when asked about the matter that Putin would be informed about Dadin's letter.

"This is a case which merits the closest attention of the relevant authorities, in this case the prison service," said Peskov.

Dadin told his wife he feared he might be murdered to shut him up.

"In the event of my sudden death, you may be told that I committed suicide, had an accident, was shot while trying to escape, or (died) fighting with another prisoner," he wrote.

"But this would be a lie."

(Additional reporting by Svetlana Reiter and Denis Pinchuk; Editing by Kevin Liffey)