A lawyer for Zhou Yongjun, a student leader during China's 1989 Tiananmen pro-democracy movement, says he was put on trial Thursday for fraud.
The opening of the trial against Zhou, a long-time U.S. resident, comes just a day after President Barack Obama wrapped up his first visit to China.
Zhou's family and lawyer say the 42-year-old is being tried in a court in China's southwestern province of Sichuan.
Zhou has been detained in China since September last year, when he tried to enter the country after living in the United States for more than a decade.
The fraud charges involve a bank in Hong Kong, but Zhou's lawyer and other supporters say the allegations are being used as an excuse to punish him for years of activism.
Zhou holds a green card in the United States, which gives him residency rights, but he is not a U.S. citizen.
Washington does not have any formal power to intervene in the case and Chinese authorities are not obligated to tell the United States about any developments.
During his four-day visit to China this week, President Obama expressed his general hopes for broader human rights in China. He did not mention specific cases and it was unclear whether he discussed Zhou's situation with Chinese officials behind closed doors.