Supporters of Lee Ming-cheh, an NGO worker who was arrested during a trip to the Chinese mainland in March, gathered outside the courthouse in Hunan province's Yueyang city, according to photos on social media.
A live broadcast of the trial on the official social media account of the Yueyang intermediate court said that Lee was being tried on charges of "subverting state power".
His wife and mother arrived in Hunan Sunday to attend the trial. Both women were accompanied by several officials from Taiwan's semi-official Straits Exchange Foundation, which handles relations with the mainland.
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.
More From This Section
After Lee went missing Chinese authorities confirmed he was being investigated for suspected activities "endangering national security".
Beijing has repeatedly ignored Taipei's requests for information on Lee's whereabouts and details of the allegations against him.
Relations between the two sides have worsened since Taiwanese President Tsai Ing-wen took office in May last year. Since then Beijing has cut off all official communications with Taipei.