"I was forced to keep my hands up, so blood cannot be pumped up my arms. It felt extremely painful." Cheng disappeared in August on a one-day business trip to Shenzhen and his lawyers were unable to find him at Shenzhen police stations. He was unable to contact his family. Loading Replay Replay video Play video Play video He emerged two weeks later as the Chinese state newspaper Global Times reported that he had been arrested for visiting a prostitute. But Cheng has now revealed on Facebook that he visited Shenzhen to collect money from the parents of a mainland Chinese person who had been arrested in Hong Kong for protesting, to help pay for living costs as they awaited trial.

Cheng said he joined the pro-democracy movement and had maintained a Lennon Wall in Jordan but had done nothing illegal. Police in riot gear move through a cloud of smoke as they detain a protester at the Hong Kong Polytechnic University. Credit:AP The British consulate had instructed staff to collect information about the protests to evaluate travel warnings for British citizens, he said. He was asked to join protester Telegram groups and approach protesters to learn about their goals. Cheng had met "a few" mainland Chinese protesters who were later arrested by police. In a development that will heighten fears about Chinese authorities operating a border checkpoint in West Kowloon inside the High Speed Rail Station, Cheng said it was this checkpoint where he was stopped by mainland Chinese police while trying to return to Hong Kong.

He was sent back to Shenzhen by train. Cheng said he was buckled to a "tiger chair" and asked about his role in the "riots" by secret police in Shenzhen. He was beaten with a sharpened baton when he was unable to do squats or chair poses for hours. "They also poked my vulnerable and shivering body parts, such as knee joint," he said. He was forced to open his iPhone when secret police grabbed his hair to submit to facial recognition and used "violence" to make him give up his passcode.

"The interrogator said: 'We suspect you are a British spy and secret agent." Loading Replay Replay video Play video Play video The regular Chinese correctional centre officers appeared shocked at how he was treated, he wrote. He was held in the Lo Wu prison, but was taken outside the centre in shackles, handcuffs and a hood to be interrogated by secret police. His interrogators told him "China is a country where it is not suitable to have full democracy ... because the majority of the population are still not well educated".

At other times he was ordered to stand still for hours and could not sleep, and if he fell asleep would punished by being forced to sing the Chinese anthem. He was slapped in the mouth if he spoke without seeking permission from his "master". He was told: "How dare you work for the British to supervise the Chinese." Cheng said he was accused of being a British spy, and secret police wanted him to "confess" and say that Britain had instigated riots in Hong Kong by donating money and equipment, and that he had organised and participated in riots. At one point his injuries were so bad from physical torture that the physical interrogation stopped, but was kept in solitary confinement.

At another interrogation centre, Cheng alleged he saw 10 young people handcuffed and heard a voice shout at them to raise their hands higher and refer to waving flags in protests. "I guess they were torturing Hong Kong protesters," he said. A secret policeman pointed to a handcuffed girl on another day and said she was "one of the scums" caught joining the protests in Hong Kong. Britain’s Foreign Secretary Dominic Raab said the mistreatment of Cheng during his detention amounted to “torture”, and he had summoned the Chinese ambassador to express “outrage at the brutal and disgraceful treatment”. China's foreign ministry on Wednesday said Cheng's legal rights were protected while he was in custody and confessed his guilt.