A Taiwanese man detained in China and accused of "endangering national security" appeared on Chinese television Sunday evening, admitting to illegally filming military exercises in a city bordering Hong Kong during protests there last year.

Relatives of Lee Meng-chu had said he went missing after crossing from Hong Kong into Shenzhen on August 19, 2019.

Hong Kong was then in the throes of months-long mass demonstrations against a law allowing extradition to China, the territory's government and the central government in Beijing.

At the time, thousands of Chinese military police had gathered in a stadium in Shenzhen for exercises with armoured vehicles.

The manoeuvres had fuelled speculation that China was going to deploy its forces to intervene in the Hong Kong unrest.

"I took my phone to record some videos," Lee said in the CCTV state television report on Sunday, interspersed with scenes from his alleged actions.

"I am sorry. I have done a lot of bad things," said Lee, his hair cropped short, wearing a blue shirt and an orange vest with his prisoner number.

According to CCTV, he went to Shenzhen for the sole purpose of observing the manoeuvres.

He reportedly shot 16 videos and took 48 photos around the stadium. After he shared some on the internet, he was reportedly arrested by state security.

Human rights organisations accuse China of regularly forcing detainees to deliver public "confessions" broadcast on television, in a country where the opaque judicial system remains subject to the ruling Communist Party.

The Chinese government has accused Taiwanese authorities and activists in recent months of supporting anti-government protesters in Hong Kong.

© AFP