A harrowing account by a Russian political prisoner of mass beatings and torture at the hands of prison guards is sending shockwaves through the country's human rights community.

In a letter published on Tuesday reportedly written by Ildar Dadin to his wife, he described his incarceration at IK-7 prison in the northwestern town of Segyezha as “hell.”

In December 2015, Dadin became the first person to be convicted under a new Russian law criminalizing “repeated violations of protest laws." He was sentenced to 2.5 years in prison.

In the letter, Dadin said prison guards planted two blades on him when he first arrived at the prison in September, in order to “find” them during a search and then lock him up in solitary confinement.

A day later, he claims, he received multiple visits from the prison director and more than a dozen guards. “They beat me four times that day, with 10 to 12 people kicking me at a time,” Dadin wrote in the letter, which was first published by the Meduza news website. “After the third beating, they pushed my head into the toilet in my cell.”

The torture continued the next day, he says, with guards handcuffing his hands behind his back and hanging him up for half an hour. “Then they took off my underwear and said they would bring in another prisoner, who would rape me, if I didn't agree to end my hunger strike.”

Dadin dictated the letter to his lawyer during a visit on Monday, who then gave it to Dadin's wife, Anastasia Zotova, she told The Moscow Times.

In the letter, Dadin said he has been too scared to put pen to paper himself. There is an “informational blockade” in the prison, he told his lawyer, and top prison staff have threatened to kill him, if he dared to complain in writing.

The prisoner asked his wife to spread the news as widely as possible to attract attention to his predicament. “If they subject me to torture, beatings, and rape again, I'll hardly last another week,” he said.

“I love you and hope someday to see you again. Your Ildar,” the letter ends.

It has not been possible to verify the authenticity of the letter. Dadin's wife told The Moscow Times that she has not been allowed to contact or see her husband since Aug. 22. “I'm in complete shock,” she said during a phone conversation. “I really fear for his life. If something happens to him, I won't survive it.”

The human rights organization Amnesty International has declared Dadin to be a political prisoner, describing his incarceration as “a shocking and cynical attack on freedom of expression.”

Later on Tuesday, the Federal Penitentiary Service's deputy director, Valery Maksimenko, told the Interfax news agency that Dadin does not have any injuries. “He himself confirms this on video,” Maksimenko was cited as saying.

Federal prison officials say they are investigating Dadin's allegations and will review video footage from the prison, the agency told the Interfax news agency.

Prominent human rights lawyer Pavel Chikov told The Moscow Times that it is too early to draw conclusions. “All we have for now, are the words of one person,” he said.

But getting to the truth of what takes place at the prison is likely to be difficult, since prison guards, doctors, and even inmates themselves would likely be in on the conspiracy. “In those conditions, it's practically impossible to prove anything objectively,” Chikov said.

Veteran human rights activist Zoya Svetova was less careful with her reaction. Practices such as those described in Dadin's letter are “common” in so-called “red prisons,” she said, referring to facilities run directly by prison authorities.

“They break people there,” explained Svetova, who until recently was a member of the Russian government's official prison watchdog, the Public Monitoring Commission.

The risks are especially big for people imprisoned on political charges. “They're in a risk zone,” she said, especially if placed in prisons far from population centers, such as Dadin's prison in Karelia.

The only way to prevent abuse is to have human rights monitors and lawyers pay daily visits, she said.

In the letter, Dadin said he did not want to be transferred to another prison, citing solidarity with his fellow inmates.

“Constant beatings, mockery, humiliation, insults, unbearable conditions — all of this also happens to other inmates,” he wrote.

According to Svetova, the publicity around the letter is likely to improve conditions at the prison by putting it under a magnifying glass.

“In that sense, the exposure caused by political prisoners can bring benefits,” she said. “They tell the story of what happens in our system.”

The Segyezha prison was also home to jailed tycoon Mikhail Khodorkovsky, who was imprisoned on fraud charges several years ago in a case widely seen as political.

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Jailed Russian Dissident Pleads for His Life in Letter Home to Wife