Three Russian prison guards have pleaded guilty to joining in the mass beating of a prisoner in a case that has revealed more than 50 complaints of torture at a single prison colony and led a lawyer representing the victim to flee the country after receiving death threats.

Gruesome video showed about 10 Russian guards taking turns to beat prisoner Yevgeny Makarov with rubber truncheons about the soles of his feet and legs as he was held face down on a table. The guards ignored Makarov’s screams and switched positions methodically, occasionally pouring a bucket of water over the convict’s head. One guard lectured Makarov for swearing at him.

“Only for those with strong nerves,” wrote Novaya Gazeta, the independent newspaper that first published the footage this month, prompting six arrests, 17 suspensions, reports of an internal hunt for the leaker, and a flood of new accusations of similar treatment.

The 10-minute video, shot by an unidentified person wearing a body-mounted camera at a prison colony near the city of Yaroslavl, was given to Novaya Gazeta by Irina Biryukova, a lawyer for the Public Verdict NGO representing Makarov.

She has since fled Russia with her family saying that she had received death threats and was unsure whether the government was ready to provide protection for her.

Facebook Twitter Pinterest Lawyer Irina Biryukova, representing Yevgeny Makarov, has fled Russia with her family after death threats. Photograph: Sergei Metelitsa/Tass

“I was convinced to give the video to Novaya Gazeta because these kinds of cases require wide publicity,” Biryukova told the Guardian on a social network site from a location outside Russia. The case “needs publicity because I don’t trust in a fair investigation in our country. But [the authorities] are scared of publicity, so it’s one of a few measures that can move this case forward from a standstill.”

Russia has one of the highest incarceration rates in the world, surpassed only among G20 countries by the United States. Russian prisoners are often interned at remote corrective colonies and periodically stage hunger strikes or engage in self-harm, like slashing their wrists with razors, to protest against harsh prison conditions.

But rarely is there video of what goes on behind the scenes and the video of the beating of Makarov, who was said to have sworn at an officer, has prompted heightened scrutiny. The agency that oversees Russia’s penitentiary system has announced it will investigate the use of violence at the prison, which activists say was widespread.

Biryukova said that she had received complaints about violence in this prison from 50 people who had been imprisoned at Corrective Facility 1 outside Yaroslavl, a city nearly 200 miles north-east of Moscow.

“We’re talking about torture that happened in relation to prisoners unconnected to our case,” Biryukova said, adding she could not go into specifics. “According to prisoners who have already been released, the use of violence was very widely practised in this prison colony.”

Biryukova revealed earlier this week that she had fled Russia after receiving death threats via an intermediary. Amnesty International has demanded that the Russian government protect her and Makarov during the investigation.

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Makarov has been transferred to another prison colony, where he has continued to be targeted by prison guards, a member of a presidential human rights council told Russian media this week.

“We remain extremely concerned for the safety of Yevgeny Makarov and the thousands of others detained in Russian pre-trial detention centres, penal colonies and police stations where allegations of torture and other ill-treatment are rife and investigations are rare,” the human rights NGO said in a statement on Monday.

Biryukova told the Guardian that while Wednesday’s arrest of the officers was an “important stage” in the investigation, it was just one among many factors that would determine when, and if, she would return to Russia.

“As long as there are heightened tensions because of the arrests, as long as I receive threats, as long as there remains no official position about my request for government protection, and if it is given, then to what degree those measures will be enough for my and my family’s safety, I can’t be safe in Russia,” she said.