The flight out of China marked the end of a harrowing ordeal for the Taiwanese businessman who had been held in the country for more than 1,400 days.

"I felt a huge relief after going through the passport check, and I cried a little," he told the BBC this week. "I have returned to the free world."

Mr Lee was arrested and jailed in 2019 after he snapped pictures of police officers in Shenzhen. He was accused of espionage and stealing state secrets - a charge he now denies.

He was released from jail in July 2021, but was prevented from leaving China as he was "deprived of political rights".

It is rare for Beijing to impose this penalty, which includes an exit ban, on convicts who are not mainland Chinese nationals. Activists say that Mr Lee's Taiwanese identity may have prompted authorities to make a political point, amid escalating tensions.

Taiwan regards itself as a self-ruled island, distinct from mainland China, with its own laws and democratically elected leader.

However, China sees island as a breakaway province that will eventually be brought under Beijing's control, by force if necessary.

China and Taiwan: A really simple guide What's behind China-Taiwan tensions? Like the thousands of Taiwanese who do business in China, Mr Lee visited the country on a work trip in August 2019. At the time he was working for a tech company.

He was no stranger to China, as he previously worked and lived in the eastern city of Suzhou, and also travelled to mainland China about twice a year.

When he visited tensions were running high because Hong Kong was engulfed in the most widespread pro-democracy protests it had ever seen. Almost every weekend, the city saw increasingly violent clashes between the police and protesters.

Curious and sympathetic to the protesters' cause, Mr Lee made a brief detour to Hong Kong, where he watched a rally from the sidelines and passed out pamphlets with messages of support. Then, he went to neighbouring Shenzhen in mainland China to meet a colleague.

At that time, hundreds of armed police officers gathered and armoured vehicles were on display at a stadium in Shenzhen. Many were worried that Beijing would send in these forces to quell the protests in Hong Kong.

The businessman spotted the activity from his hotel room window, so he walked over to the stadium and took some photos. He said there were no warning signs and he didn't cross the police cordon. Many others were also photographing the scene, he said.

Mr Lee denies he was spying. "I am only a curious passer-by… if it really were some state secret, how could everything be seen from a hotel?"

The Shenzhen gathering of police forces was photographed by many people, including news agencies When he was departing Shenzhen, ten video cameras he was transporting back to Taiwan for his business caught the attention of airport officials.

They stopped him to search his luggage and his phone, and found his pamphlets as well as the photos of police forces at the Shenzhen stadium.

National security officers then brought him to a hotel to undergo "residential surveillance at a designated location". For 72 days, he was not allowed to leave his room and watched by three people every day. He wasn't allowed to watch TV, read newspapers, open the curtains or even speak.

"I was actually looking forward to their questioning every day, or otherwise no one was willing to speak to me,"Mr Lee said. "Every day I had nothing to do so I just cleaned the floor, under the bed and the ceiling. It was painful."

Activists say Beijing often uses this secretive and arbitrary form of detention against those accused of national security offences. They can be held for months without trial.

Mr Lee was then whisked off to a detention centre, and only resurfaced months later.

He appeared on state broadcaster CCTV saying he felt sorry for "doing some harm to the motherland".

Mr Lee told the BBC he apologised in the hopes that he would be released as soon as possible. "You couldn't be bothered by things like dignity."

But soon after, he went on trial and was sentenced to one year and ten months in jail for "foreign espionage and illegally sending state secrets".

Chinese state media ran extensive reports about his case, alleging he had taken the pictures of the Shenzhen stadium to send to Taiwanese groups.

They also cited the fact that he had studied in the US and was a member of Taiwanese non-governmental organisations to allege he was a Taiwan independence activist, which Mr Lee denies.

Mr Lee served his sentence in a Guangdong jail, where he was crammed into a small cell with 15 other prisoners. But for him, prison was an improvement from residential surveillance - at least he had company.

He was put to work in a production line and had to wrap computer cables every day. If they failed to finish their tasks on time, they would be physically punished, he said.

China's Taiwan Affairs Office has not responded to the BBC's questions. The BBC has not been able to independently verify all of Mr Lee's claims, but his account of his time in detention is similar to those shared by other detainees.

Taiwan sees MeToo outpouring after Netflix show How China calibrates its Taiwan response During his trial Mr Lee had been sentenced to "deprivation of political rights". At the time he did not give it too much thought, he said, as he did not see himself as a Chinese citizen in the first place.

But a month before his scheduled release, he was shocked to find out that he couldn't leave the mainland for another two years.

Yaqiu Wang of Human Rights Watch said that in Mr Lee's case, "the Chinese government wanted to make a point that he's a Chinese citizen".

It is difficult to ascertain the number of Taiwan-linked individuals arrested in China for national security offences. However, it is "reasonable" to assume the number is increasing amid worsening relations between Beijing and Taipei, she said.

In April, Taiwan-based publisher Fucha, who often printed books critical of Beijing, was held for an investigation for endangering national security. Earlier that month, Taiwanese activist Yang Chih-yuan was charged with secession.

The difference in the Chinese authorities' treatment of Mr Lee compared to previous cases may also be a sign that they are getting tougher on Taiwanese detainees.