“He is currently in good physical condition,” Ma said without providing details about where the activist was being held.

Li’s wife, Li Ching-yu, said that her husband had only engaged in advocating for rights, which she described as a basic democratic value around the world.

He is the first Taiwanese rights activist in the past decade or so to be arrested on the mainland over “security” issues.

“We demand to know on what actual charges Li Ming-che is being held, and where he is being investigated,” she said in a news conference in Taipei, hours after Beijing confirmed that the activist was in custody.

She said she suspected her husband’s concerns about rights on the mainland and his discussion of Taiwan’s democracy through his and other mainland websites were the main reasons he had been arrested.

She said his Wechat account was shut down by the mainland side last year, but that he opened another account to remain in contact with his friends.

Some of the books he sent to his mainland friends were seized in August, suggesting he might have been under surveillance by relevant authorities by that time.

The books he had sent include the Chinese translation of Mao: The Unknown Story by Jon Halliday and Jung Chang, and This Changes Everything: Capitalism vs. the Climate by Naomi Klein. Another one was about events leading from the first Sino-Japanese war in 1894 to the Japanese invasion of China in 1937.

“He was supposed to meet friends from the chat group in addition to looking for prescriptions for treatment of his mother-in-law, who has cancer,” Li Ching-yu said.

An author friend of the activist on the mainland had arranged to pick him up from the Zhuhai customs checkpoint on March 19, but failed to see him after waiting for four hours, Li said.

Asked if she planned to go to the mainland to visit her husband any time soon, she said the authorities there had to first reveal where Li Ming-che was being held, and grant her request for a visit according to the law. “The Chinese government must allow relatives of Li Ming-che to visit him in line with the law, if he is under detention or arrest as the Taiwan Affairs Office has claimed,” Li Ching-yu said, adding that such a visit was very important because her husband had high blood pressure and needed medication.

Taiwan’s Mainland Affairs Council expressed deep regret over Beijing’s failure to inform either the Taiwanese authorities or Li Ming-che’s family about his detention until Wednesday, despite repeated requests for information from the Taiwanese side since March 19.

“The mainland side must immediately make public where Li Ming-che is being detained, and what law he has violated leading to the restriction of his personal freedom,” it said in a statement, adding Beijing should also speedily arrange for a visit to the activist by his family, in line with an agreement on joint cross-strait efforts to fight crime. The council also urged the mainland side to take note of Li’s health condition and ensure his safety, and said it would help his family arrange his return as soon as possible.

Li’s case came to light around the same time as Feng Chongyi, a China-born professor based in Australia who has often criticised Beijing’s crackdown on political dissent, was barred from leaving the mainland and detained.