Simon Cheng, a Hong Kong citizen, claimed he was shackled to a steel “tiger chair”, hung spread-eagled on a “steep X-Cross” and beaten while he was detained for 15 days in August.
British Foreign Secretary Dominic Raab said Cheng’s allegations are credible and that the treatment “amounts to torture”.
Raab told BBC radio that he summoned the Chinese ambassador to denounce the “disgraceful” and “outrageous behaviour from the authorities in China” which violate international law.
Cheng was placed in administrative detention after visiting the neighbouring southern Chinese city of Shenzhen in August.
He said in a statement on Facebook that he had taken a high-speed train to Hong Kong’s West Kowloon station, where he was stopped by mainland police and sent back to Shenzhen.
Cheng said police tied him to a steel “tiger chair,” accused him of being a British spy and forced to do “extreme strength exercise” for hours, beating him whenever he failed to complete it.
He was also shackled to an X-shaped frame that kept his hands aloft for “hours after hours,” Cheng wrote.
“It felt extremely painful.”
Cheng said he was asked if he knew anyone who worked for British intelligence agencies, what part he had played in protests and what he knew about mainland citizens who had joined the demonstrations.
China has repeatedly accused Washington and London of condoning violence in Hong Kong.
The former British colony was handed back to Beijing in 1997 under a 50-year agreement that gave Hong Kong special rights unseen on the mainland, including freedom of speech and an independent judiciary.
The protests started as a response to a now-scrapped extradition bill but has since expanded to include broader demands for democracy and investigations into police violence.
Chinese foreign ministry spokesman Geng Shuang on Wednesday said that Shenzhen police had “guaranteed” Cheng’s “legitimate rights and interests in accordance with the law.”
Geng said that he was unaware of any statements from London on Cheng’s situation, but that China expressed “strong indignation at the recent series mistakes and deeds of the British side on the Hong Kong issue.”
Police in Shenzhen declined to comment on Cheng’s statement when contacted by AFP.