The interrogators work in secret, barraging members of China's Communist Party they accuse of corruption with questions, shouts and more severe measures. Suspects are kept from sleeping, deprived of adequate food and water, and exposed to extreme temperatures. They are forced to stand for hours or sit on painfully small chairs and, in some cases, beaten with steel rebar. Often their families are threatened. Sometimes, their money is extorted.

For those caught up in China's feared shuanggui system, there is no right to call a lawyer or family, no right to see evidence presented against them, and effectively no limit to how long they can be detained.

Now, human rights activists are calling on China to shut down the decades-old system, after a lengthy new report accuses the country of employing torture to wage a war on corruption that has swept up hundreds of thousands of Chinese people in recent years.

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"We feel it's urgent that China abolish the system to try to minimize some of these horrific abuses, and what are probably pretty shaky prosecutions," said Sophie Richardson, China director at Human Rights Watch, which released the report in Hong Kong on Tuesday.

The report also underscores questions about the nature of evidence produced from the Chinese justice system, a particularly important question for Canada as it begins talks toward an extradition treaty with China, which is seeking the repatriation of people it calls corrupt.

In China, where the justice system relies heavily on confessions, the primary evidence for corruption often comes from the shuanggui system, which conducts internal disciplinary matters for China's Communist Party and exists outside the bounds of the law. For Chinese leaders, it has been a useful tool in a sweeping war on corruption waged under President Xi Jinping, generating valuable confessions that can be forwarded to courts and used to place people behind bars.

But much of its work is kept out of the public view – even offices used for interrogations are unmarked or located in hostels and private homes; one was located behind a sign for a "Literature Society," Human Rights Watch found.

Some have emerged from those offices with horrifying accounts. In 2014 former municipal land bureau director Zhou Wangyan posted a detailed description of being suffocated with over a dozen lit cigarettes stuffed into his nose and mouth. He also described a tactic like waterboarding, where interrogators forced his face into a sink full of water, making him feel as if he was about to drown. They also beat him.

"They viciously whipped the bottom of my feet with a 4-6 mm steel rebar or a bundled iron wire," he said in an account he posted to social media. "I fainted twice."

Another time, "they made me lie on a bench under which there is a pot of lit charcoal; then when I was so roasted that I was sweating all over, they poured cold water on my head; the cold water dripped into the brazier and immediately became steam, choking me and leaving me out of breath."

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The shuanggui system was created in China in the 1990s – its name in Chinese, "double designation," refers to calling people to appear at a "designated location at a designated time."

It has no analogues in modern Western justice systems, which can make it difficult for outsiders to grasp the way it functions outside the law, and the fear it instills inside the country.

"It seems like something out of another century or two centuries ago. It's quite shocking," said David Mulroney, the former Canadian ambassador to China who is now president of the University of St. Michael's College at the University of Toronto. The use of the shuanggui system, he said, should inform the Canadian discussion about the wisdom of pursuing an extradition treaty with China.

"It's a particularly bad example of a system that is still flawed," he said.

"What China tries to do is to make this a Canadian problem, saying, 'you have all these bad guys from China, therefore we have to come up with this mechanism to get them back.' When in reality, it's a Chinese problem that in some sense is landing on our shores. And the way to address this is through improvements in China," he said, rather than asking Canada to establish closer ties with a system "that may violate our own laws and values."

The problems go beyond the reliability of evidence. By keeping the shuanggui system outside the legal process and shrouded in secrecy, alleged criminals in Canada also struggle to argue they will be mistreated if they are sent back.

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"We tell people you need to show that it is likely you will be tortured if you are returned. But it is extremely difficult to show the courts what the system is that you will be returned to, because that system is opaque," said Margaret Lewis, a law professor at Seton Hall University School of Law who studies China's legal system.

"I don't know what kind of assurances the U.S. or Canada or other Western governments could be given that would be adequate to give comfort that the system was living up to the requirements of the Convention Against Torture."

China has promised reform to its shuanggui system. Jiang Mingan, a law professor at Peking University who was consulted on possible changes in 2013, said shuanggui would likely be abolished in the long-term.

"The duration of shuanggui will certainly be shortened in the near term," he told the South China Morning Post three years ago. "The procuratorate will take a more important role in collecting evidence of the crimes."

Chinese legal scholars have called the shuanggui system unconstitutional.

But statistics published in Chinese media have shown, instead, a huge increase in internal party investigations in recent years, as Mr. Xi wages a corruption campaign that blends anti-graft work with efforts to vanquish potential rivals.

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Last year, 330,000 cases were investigated by local Commissions for Disciplinary Inspection, up nearly 50 per cent from 2014, according to local media reports, and more than double the figure in 2011.

Of the many tools those investigators hold is the ability to isolate suspects and produce a sense of hopelessness unless they admit guilt, Human Rights Watch found.

In the shuanggui system, "you're being held incommunicado and ill-treated for the explicit purpose of getting you to confess. You have no recourse," Ms. Richardson said.

One of the four people who spoke with Human Rights Watch about their personal experience in shuanggui was held for 14 months. (The organization interviewed 21 people, including lawyers and family members, and reviewed a further 73 cases that were either reported in Chinese media or found in a database search of court documents posted online.)

Interrogation facilities are specially designed to prevent people from attempting suicide.

Ms. Richardson likened the system to mafia justice, visited by the Communist Party on its own members. "The Sopranos, I think, is the appropriate comparison."

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The families of some suspects are asked to pay money as a gesture of goodwill, only to have that payment be used against them as evidence of corruption by procurators who label it the repayment of the proceeds of graft.

Others who complain in court that they were tortured are told by judges that they cannot complain about a system that exists outside the law – only to then have those same judges rely on confessions obtained in shuanggui.

"It's this completely Kafkaesque system," Ms. Richardson said.

One woman told Human Rights Watch that she tried to assert her rights as a citizen to the investigators on her case. "A CDI officer said to her, 'we are the CDI, we are not bound by the law. We can do what we want,'" Ms. Richardson said.