Lee Ming-cheh appeared alongside a Chinese co-defendant named Peng Yuhua at Yueyang intermediate people's court in central Hunan province, according to video footage posted on the court's official social media account.
A shaven-headed Lee appeared nervous as he confessed to charges of "subverting state power", stating that he had written and distributed online articles that criticised China's ruling Communist Party and promoted democracy among other topics.
"I know that my behaviour definitely violated Chinese law," said Lee, an NGO worker who was arrested during a trip to the Chinese mainland in March.
"This mistaken thinking led me to violate the law. I express my guilt and regrets."
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The articles were written at the direction of Peng, who he had become acquainted with online and had met several times on the Chinese mainland, Lee said under questioning from a prosecutor.
Taiwan's presidential spokesman Alex Huang said Taipei was "highly concerned" about Lee, including "his health and the rights he's entitled to."
"The government's consistent stance is to ensure his personal safety and to make every effort we can to bring him home safely, under the premise of safeguarding our nation's dignity," Huang said.
His wife and mother travelled to Hunan for the trial and his supporters gathered outside the courthouse in Yueyang city.
Both women were accompanied by officials from Taiwan's semi-official Straits Exchange Foundation, which handles relations with the mainland.
After the trial, his wife, Lee Ching-yu, showed reporters permanent tattoos on both her forearms that read, "I am proud of you, Lee Ming-cheh."
"I knew I wouldn't be able to bring anything into the courtroom with me, except for the expression in my eyes," she said, her voice wavering.
"The world and I today witnessed this big political show, and also witnessed the difference between Taiwan's core beliefs and values and that of China," she said.
Lee has long supported civil society organisations and activists in China, according to Amnesty International.
He had shared "Taiwan's democratic experiences" with his Chinese friends online for many years and often mailed books to them, said the Taiwan Association for Human Rights.
The language the two co-defendants used to confess their crimes sounded rehearsed, according to Patrick Poon, a China researcher at Amnesty International.
The "language they used was so much like the Chinese government's", he said, calling it a "sham trial."
Frances Eve, researcher for the charity Chinese Human Rights Defenders, said the trial's "phoney transparency" was an attempt to deflect attention from the serious denial of Lee's due process rights.
"He was held for months in a secret location and had already 'confessed' before the trial, according to state media. There is a strong likelihood that he was tortured to force a confession," Eve told AFP.
Relations between the two sides have worsened since Taiwanese President Tsai Ing-wen took office in May last year, with Beijing cutting off all official communications with Taipei.
China sees self-ruled Taiwan as part of its territory waiting to be reunified. It wants Tsai to acknowledge the island is part of "One China", which she has refused to do.
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