China is accused of detaining up to 1 million Uighur Muslims in internment camps in the western region of Xinjiang.

Two Uighurs who had been inside those camps told BBC Newsnight about the physical and psychological torture that goes on.

One described being shackled to a chair, deprived of sleep, and beaten by authorities.

Another said he saw people he used to know look like they had "lost their souls."

New details have emerged of the reeducation camps in which China has been accused of imprisoning up to 1 million Uighurs, a majority-Muslim ethnic minority, in the country's western region of Xinjiang.

Political prisoners in those camps are both physically and mentally tortured, two eyewitnesses of the camp told BBC Newsnight in a documentary that aired Thursday night.

One, named Azat, had been in a detention centre to visit a detainee, while the other, Omir, had been imprisoned in one of the camps. Both are Uighurs and have since fled Xinjiang, the BBC said. The broadcaster did not disclose Azat's or Omir's current locations because they feared retribution from the Chinese government.

Omir, who was detained in Karamay, in north Xinjiang, described being shackled to a chair, deprived of sleep, and beaten by police in his camp.

He told the BBC:

"They have a chair called the 'tiger.' My ankles were shackled, my hands locked to the chair. I couldn't move. They wouldn't let me sleep. They also hung me up for hours, and they beat me.

"They had thick wooden and rubber batons, whips made from twisted wire, needles to pierce the skin, pliers for pulling out your nails.

"All these tools were displayed on the table in front of me, ready for use at any time. You could hear other people screaming as well."

Omir described being tortured by Chinese police at an internment camp in Karamay, Xinjiang. BBC Newsnight

Omir added that he was later moved to another internment camp, where he was forced to share a small room with 45 other people. They took turns sleeping because there was so little space, he told the BBC.

He said he ended up in a camp after police accused him of aiding Islamic extremists — an allegation he has denied.

China justifies its surveillance and crackdown in Xinjiang as preventing terrorism, and has repeatedly accused militant Uighurs of starting terrorist attacks across the country since at least the mid-1990s.

Omir's account squares with previous reporting on the camps, such as that by Simon Denyer of The Washington Post published this May. Kayrat Samarkand, another Uighur who had been imprisoned in a reeducation camp, also described being strapped in the "tiger chair" and being waterboarded if he disobeyed orders.

Read more: Why the Muslim world isn't saying anything about China’s repression and 'cultural cleansing' of its downtrodden Muslim minority

Law-enforcement officials patrol near the Id Kah Mosque in Kashgar, Xinjiang. AP Photo/Ng Han Guan

The torture in China's camps appears to go beyond the physical. Azat, who had visited someone detained in a camp, described seeing detainees literally forced to sing propaganda songs to get food, and watching people he used to know look like the "lost their memory."

Azat, whose face and voice were obscured for his protection, told the BBC: "It was dinnertime. There were at least 1,200 people holding empty plastic bowls in their hands. They had to sing pro-Chinese songs to get food."

He told the BBC added that he recognized people he used to know among the prisoners, saying:

"They were like robots. They seemed to have lost their souls. I knew many of them well — [we] used to sit and eat together.

"But now they didn't look normal to me. They behaved as if they weren't aware of what they were doing. They were like someone who had lost their memory after a car crash."

Azat's observations are consistent with those cited in The Washington Post's May report, which said that a typical day in an internment camp in Karamagay, another village in Xinjiang, included studying President Xi Jinping's political dogma, singing about the Chinese Communist Party, and chanting phrases like "Long live Xi Jinping."

Uighurs in Kashgar, Xinjiang, carry a flag of the Chinese Communist Party and walk past a billboard with President Xi Jinping's face on it in June 2017. Kevin Frayer/Getty Images

The Chinese government has denied that internment camps exist, but have acknowledged a program of "resettlement" for people it refers to as extremists. It also told the BBC that the Xinjiang government "has taken lawful and effective measures including education and training to prevent the infiltration and spread of religious extremism."

The UN on Thursday called on China to "halt the practice of detaining individuals who have not been lawfully charged, tried, and convicted for a criminal offense in any extra-legal detention centre."

A group of 17 bipartisan lawmakers in the US also wrote an open letter on Wednesday calling on Secretary of State Mike Pompeo and Treasury Secretary Steve Mnuchin to apply sanctions against China for its repression on the Uighurs.