Russian authorities say they have dismissed four prison officials over allegations of rape and torture.
A video was released this week by an NGO claiming to show a prisoner being sexually abused in central Russia.
The NGO Gulagu.net said it had received hundreds of reports of systematic torture in Russian prisons.
The Russian Federal Prison Service (FSIN) confirmed that it had sacked the director of a prison hospital in Saratov, as well as one of his assistants and two other prison officials.
In a statement, FSIN said that investigators had been sent to "verify information about misconduct" and alleged violence against inmates at the prison facility.
The authority added that the four individuals had been "released from their positions and dismissed for negative reasons".
"All perpetrators of the crimes committed are held accountable to the fullest extent of the law," the FSIN said.
Video evidence leaked
On Monday night, Gulagu.net posted a nearly three-minute video on its Telegram account of the apparent rape of a Russian prisoner.
In the footage, a naked man can be seen on a bed with his hands and legs tied, while another man abuses him with a long pole.
A third man films the incident while another individual holds a cloth over the victim's face to muffle his screams.
According to Gulagu.net, the rape took place in February 2020 at a medical unit for prisoners with tuberculosis in Saratov.
The NGO says that an anonymous Belarusian whistleblower has sent them over 1,000 videos showing torture in prison.
Gulagu.net founder Vladimir Osechkin said on Facebook that the videos "prove" that members of the FSIN and Russia's Federal Security Service (FSB) use rape and other forms of torture to force inmates to cooperate.
"It is the first time that human rights defenders have obtained such a colossal amount of information proving the systemic nature of torture in Russia," Osechkin told AFP.
The NGO stated that it would release more videos of alleged prison abuse in the coming days.
"A serious investigation"
The regional branch of Russia's Investigative Committee said on Wednesday that it had launched several investigations into "sexual violence" and "abuse of power".
The country's General Prosecutor's Office is also reportedly probing the matter.
According to the Investigative Committee, four alleged rapes were committed against inmates between January 2020 and May 2021 in the Saratov medical prison.
"If the authenticity of this material is confirmed, it would be grounds for a serious investigation," added Kremlin spokesman Dmitry Peskov.
Sexual violence is punishable in Russia with up to 10 years in prison. The country's prison system is regularly marred by torture scandals, committed by guards or other inmates.