The number of Chinese companies exporting "tools of torture" has surged over the past decade, Amnesty International said Tuesday, with many devices falling into the hands of rights violators worldwide.

More than 130 Chinese firms now produce electric shock stun batons, spiked batons, weighted leg cuffs and other "potentially dangerous law enforcement equipment", up from 28 in 2003, the UK-based rights campaign group said in a report co-authored with Omega Research Foundation.

One company -- state-owned China Xinxing Import and Export Corporation, whose products include thumb cuffs, electric shock guns and restraint chairs -- had more than $100 million in trade with African countries as of 2012, according to the report.

"China appears to be a leader in the less savoury side of the so-called 'tools of torture' -- equipment that we at Amnesty believe is intrinsically cruel," said Patrick Wilcken, Amnesty's security trade and human rights researcher and lead author of the report.

China's own justice system remains riddled with abuses, campaign groups say, with confessions extracted through torture not uncommon.

While there are few legal prohibitions on its manufacture and trade in China, Wilcken said, such equipment often ends up being sent to "very unsafe and risky situations" across the globe.

"What we've found is that it appears the Chinese authorities do not have a kind of rigorous vetting process in terms of where this equipment is exported to," Wilcken said. "They're not doing risk assessments."

Amnesty is "calling on not just China but every country to bolster their regulations on the trade in this equipment, so that licences for trade in situations where there's a high risk for violation should not be issued," he added.

The Amnesty report, entitled "China's Trade in Tools of Torture and Repression", examines some tools that the organisation calls "inherently abusive".

Among them are spiked batons, considered "specially designed implements of torture" by the US Bureau of Industry and Security. China is the world's only known manufacturer of them.

Electric shock stun batons can be used to cause injury to sensitive body parts including the groin and neck, often leaving no physical trace, the report notes. Amnesty has denounced their use due to the "substantial risk" of abuse.

Weighted leg cuffs are designed to cause the wearer discomfort, while some thumb cuffs advertised in China have serrated edges that can cut the wearer if tightened, according to Amnesty.

The use of rigid restraint chairs, which can include painful metal or wooden restraints, has been denounced by the UN Committee against Torture.

In addition to these devices, the Amnesty report also examines several types of "legitimate" law enforcement tools such as tear gas and anti-riot equipment that have nonetheless been exported from China to countries "where there was a foreseeable risk of serious human rights violations", including Uganda and the Democratic Republic of Congo.