An ethnic Mongol dissident who spent almost 20 years behind bars in China has been freed, Beijing said Wednesday, as he reportedly accused authorities of torturing him in jail.

Hada became one of China's longest-serving political prisoners after being sentenced in 1996 for "espionage" and "separatism" when he advocated greater freedoms for China's six million Mongols.

He was not released when his 15-year jail term ended and was instead transferred to a detention centre in the northern region of Inner Mongolia.

Authorities said he also had to serve a separate four-year sentence for "deprivation of political rights," a loose phrase often used as a justification to extend detentions.

"The punishment of four years of deprivation of political rights imposed on Hada finished on December 9," foreign ministry spokesman Hong Lei said in Beijing.

"The Chinese government has dealt with this case in accordance with law," Hong told a regular briefing. "This is totally legal and justified."

Hada told US-funded broadcaster Radio Free Asia (RFA) he had been abused while in prison and that police had imposed conditions on him after his release Tuesday.

"After so many years of torture, my body is riddled with disease, and all for what was a miscarriage of justice in the first place," Hada RFA after his release.

"I have been diagnosed with more than 10 different conditions, all of them the result of the torture, physical punishments and maltreatment I suffered in jail."

Hada, who is reported to be in his fifties, was detained after writing essays on greater Mongol autonomy and organising peaceful demonstrations.

The veteran activist will likely be kept under surveillance along with his family, who rights groups say has suffered harassment from authorities.

Some of China's ethnic Mongols, who have cultural and ethnic ties with Mongolia, complain of political and cultural repression under Chinese rule -- a charge Beijing denies.

The ruling Communist Party says it grants autonomy for China's minorities, but keeps a firm grip on regions which are prone to ethnic unrest such as Tibet and Xinjiang.

Inner Mongolia also sees occasional incidents. Protests hit the region in 2011 over exploitation of its resources and Chinese rule.