Dissident Jiang Qisheng, who urged students to light candles to commemorate those killed in the 1989 pro-democracy protests he helped lead, has been detained by the Chinese police in Beijing.
Mr Jiang, who was a doctoral candidate at the People's University when he helped lead mass demonstrations at Beijing's Tiananmen Square in 1989, spent more than a year in prison after the troops moved in.
He has been among China's most determined activists for democracy in recent years. Last week, he wrote letters to the government protesting against the alleged police beating of fellow activist Cao Jiahe.
The government has recently stepped up its surveillance of dissidents to prevent June 4 anniversary commemorations of Tiananmen Square.
A group of academics calling for reforms in a letter to President Jiang Zemin and Premier Zhu Rongji earlier this week demanded Ms Jiang's release. They claim they have the right to commemorate the victims of Tiananmen.
Scientist Xu Liangying and academics Ding Zilin and Jiang Peikun were among those who signed the letter.