A growing number of Chinese companies are producing, advertising and exporting tools of torture to countries with poor human rights, Amnesty International and the Omega Research Foundation have said.
According to a report by the two groups, there are more than 130 Chinese firms that market "intrinsically cruel and inhumane" devices, including electric shock stun batons, metal spiked batons, and weighted leg cuffs.
A decade ago, only 28 such companies were involved in the trade that Amnesty says is fuelling human rights violations across Africa and Asia.
"This trade – which causes immense suffering – is flourishing because the Chinese authorities have done nothing to stop companies supplying these sickening devices for export or to prevent policing equipment falling into the hands of known human rights abusers," says Patrick Wilcken, security trade and human rights researcher at Amnesty International.
Most of the companies are state-owned and sell their products to law enforcement agencies worldwide, including to some that persistently abuse human rights, the report says.
China is the only country in the world known to manufacture spiked batons, devices that Amnesty says are "specifically designed as implements of torture" and have reportedly been used by police in Cambodia and exported to security forces in Nepal and Thailand.
Some 29 companies market electric stun batons. "These devices make it easy for security officials to apply extremely painful multiple shocks by hand to sensitive areas of the body including the genitals, throat, groin or ears without long-lasting physical traces," the report reads.
Amnesty says it has also documented the use of electric shock batons in China.
One Chinese torture survivor told the group: "They [police] used an electric baton on my face – it's a kind of torture the police call "bengbao popcorn" because your face splits open and looks like popped corn. It smelled horrible, the smell of burning skin."
Wilcken says: "There is no excuse whatsoever for allowing the manufacture and trade in equipment for which the primary purpose is to torture or inflict cruel, inhuman or degrading treatment on people. These heinous acts are totally banned under international law and Chinese authorities should immediately place a ban on the production and trade in such cruel and inhumane devices."
According to the report, Chinese-made equipment was sold to security forces in Uganda and the Democratic Republic of the Congo despite fears of police brutality being used to violently quell protests in the two countries.
The Chinese Ministry of Foreign Affairs has dismissed the report, saying it probably contained false information.
"This international organisation is always biased against China and I really doubt the authenticity of the report that has been released," spokeswoman Hua Chunying said.