MOSCOW -- A jailed Russian opposition activist has accused a prison warden of overseeing the systematic torture of inmates and of threatening to kill him, allegations made in a desperate letter to his wife that he asked her to publish to save his life.

Ildar Dadin was sentenced to a three-year term in December 2015, becoming the first Russian to be imprisoned under controversial legislation that criminalizes repeated breaches of public-assembly rules, such as demonstrating in a group without the prior consent of the authorities. He is now serving his sentence at the IK-7 prison in Segezha, in the northwestern Karelia region.

In the letter to his wife, Anastasia Zotova, Dadin wrote that prison authorities planted two blades on him shortly after he arrived at the prison on September 10 so that they could then "find" them during a search, providing a pretext to put him in an isolation cell as punishment.

"This is an everyday practice -- they do this to make sure to put the new arrivals in the punishment cell so that they immediately realize what a hell they've ended up in," Dadin wrote in the letter, which was dictated to a lawyer and published by the Latvia-based website Meduza on November 1.

Dadin wrote that prison authorities then took away his soap, toothbrush, toothpaste, and toilet paper, prompting him to declare a hunger strike. The following day, he wrote, prison warden Sergei Kossiyev and three other officials paid him a visit.

"Together they started beating me," he wrote. "Over the course of the day, they beat me up four times, with 10-12 people kicking me simultaneously. After the third beating they put my head in the toilet in the punishment cell."

On September 12, they bound his hands behind his back, hung him from handcuffs and left him hanging for 30 minutes, he wrote. "Then they took off my underwear and said that now they'll bring another prisoner and that he'll rape me if I don't stop my hunger strike."

Meduza said the letter was written on October 31 and dictated to a lawyer, Aleksei Liptser, apparently to bypass what Dadin described as a "real information blockade" by which his letters are intercepted by prison authorities. He asked Zotova to publish it and ensure wide exposure, saying, "It will increase the chances I survive."

"I ask you to publish information about the fact that Major Kossiyev is directly threatening murder for attempts to complain about what is happening," he said.

'Special Means'

Russia's federal penitentiary service, known as FSIN, confirmed in comments to the newspaper Novaya Gazeta that "physical force" was used against Dadin on September 11, suggesting that he provoked it.

"Dadin in a rude way refused to come out of his cell and assume the position for a search, and began to grab [prison] employees by their uniforms, as a result of which physical force and special means were used," the newspaper quoted the prison service as saying.

It did not describe the "special means," a term that can refer to items such as tear gas and tasers but in this case might refer to batons or nightsticks.

Hours after Dadin's letter was published, several dozen people protested outside FSIN headquarters in Moscow to voice anger over the alleged abuse. Some held signs that read: "Stop the torture. Free Dadin."

Amnesty International called for Dadin's immediate release and urged the Russian authorities to "end the pattern of impunity for torture and other ill-treatment."

"Ildar Dadin's allegations of beatings, humiliation, and rape threats are shocking, but unfortunately they are just the latest in a string of credible reports indicating that torture and other ill-treatment are being widely used in the Russian penal system with impunity, with the aim of silencing any form of dissent," Sergei Nikitin, director of Amnesty International Russia, said in a statement.

Cracking Down On Dissent

The federal Investigative Committee said that it was conducting a "preinvestigation check" and that prosecutors had been sent to the prison to look into the allegations.

Kremlin spokesman Dmitry Peskov said the allegations deserved "very close attention" and that Russian President Vladimir Putin would be informed about the matter.

The legislation under which Dadin was convicted is one of several antiprotest laws signed by Putin since he returned to the Kremlin for a third term in May 2012, amid a wave of opposition protests that were the largest since he was first elected in 2000.

Rights groups have criticized the laws and other measures they say have severely restricted the freedoms of speech and assembly under Putin, who is eligible to seek another six-year term in 2018.

"No one should be in jail for peacefully expressing their opinion," Amnesty International said in its statement.

Former oil tycoon Mikhail Khodorkovsky was serving his sentence at IK-7 in Segezha when he was pardoned by Putin and flown out of Russia in 2013.

Dadin said that he did not want to be transferred to another prison.

"I've seen several times and heard how they beat up the other prisoners, so my conscience does not allow me to run away from here -- I intend to fight to help the others," he wrote.

He appealed to his wife to hire a lawyer who could be permanently stationed in Segezha, 300 kilometers from the Arctic Circle, to provide legal assistance.