Chinese police carry out "appalling" torture of criminal suspects, campaigners said Wednesday, as they released a report detailing Beijing's failure to combat abusive interrogations.

Despite China saying it has addressed the issue of forced confessions through a series of reforms, prosecutors and judges "ignore clear evidence of mistreatment" while police are covering up abuse, the report from Human Rights Watch (HRW) said.

The New York-based campaign group said police manipulate video interrogations so that confessions are made on camera, while torture -- using methods that leave no visible injuries -- takes place out of sight.

The use of ruthless "cell bosses" -- fellow detainees who oversee detention centres for the police -- was also widespread, the report said.

Suspects are sometimes strapped into metal "tiger chairs" for days, deprived of sleep and food as their legs and buttocks become swollen, according to the report, titled 'Tiger Chairs and Cell Bosses: Police Torture of Criminal Suspects in China'.

"We heard appalling stories of detainees being hung by the wrists, shackled for years, and terrorised by cell bosses, yet having no real means to hold their tormentors to account," Sophie Richardson, HRW's China Director, said in the report.

"Police continue to be expected to produce confessions in order to secure a conviction," Richardson added at a press conference in Hong Kong on Wednesday.

"The expectation of a conviction is one of the main drivers of this kind of abuse," she said.

The group analysed hundreds of newly published court verdicts, and interviewed 48 recent detainees, family members, lawyers, and former officials.

Of the 432 verdicts that made reference to torture allegations, only 23 resulted in the court throwing out evidence, and there were no acquittals, the report said.

HRW found only one successful torture prosecution concerning three police officers, but none served prison time. It also found evidence that health workers and lawyers were unwilling to assist torture victims with their complaints.

- 'Not even close to a fair fight' -

After a series of high-profile police brutality cases in 2009 and 2010, China vowed to crack down on abuses and revised its Criminal Procedure Law.

The new measures were intended to strengthen detainees' rights to access lawyers and to ban confessions and written statements obtained through torture.

The ruling Communist Party claims to have made the "rule of law with Chinese characteristics" one of its top priorities.

But the massive challenge authorities face tackling abuse was highlighted last December when state media admitted forced confessions were "not rare" in the country.

A total of 99.93 percent of criminal defendants in China are found guilty, official figures show.

"We have been making efforts to improve the legal system with the purpose of ensuring that people will have justice and fairness in each and every case," Chinese foreign ministry spokeswoman Hua Chunying said Wednesday afternoon in response to the report at a regular press briefing.

"Chinese law prohibits in explicit terms confession through torture," Hua added.

The HRW report calls on China to transfer the management of detention centres from the police to the justice ministry and to free the judiciary from political control.

"It's not even close to being a fair fight -- the police hold so much power not just relative to the suspects but relative to the courts -- that the defence is so weak from the very beginning of any given case," Richardson said, adding that defendants have very limited access to lawyers or families.

"Unless the police have their powers dramatically circumscribed in a number of different ways, we're not going to see real change."

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