Shen Tong, a student pro-democracy leader arrested after returning to China to organize a human rights group, has been charged with "engaging in illegal activities," his mother said Thursday.

In a telephone interview, Li Yixie said an official from the Beijing Public Security Bureau told her Thursday morning that Shen had been formally charged.Li said the official also told her that Shen had been moved from a hotel to an "assigned living location" but that he was "not with other common criminals."

Li went to a local police station Thursday to protest Shen's detention but said she was told she could not see him because he had engaged in "illegal activities."

Spokesmen at the Beijing Intermediate People's Court and the Beijing Public Security Bureau said they were unaware of any charges against Shen.

At a weekly news briefing, Foreign Ministry spokesman Wu Jianmin said only that "the relevant Chinese authorities are investigating his case according to law."

Shen was arrested Tuesday at his mother's apartment while preparing to announce the establishment of a Chinese branch of the human rights organization he formed while in exile in the United States.

Shen, 24, had fled China after the Tiananmen Square pro-democracy protests he helped organize were crushed. He returned last month, the first such dissident to return since the crackdown.

Arrested with Shen were Qi Dafang, who was also active in the protests, and Qian Liyun, the wife of another student leader, Xiong Yan. No word has been heard of the two since Tuesday.

On Wednesday, China expelled Ross Terrill, a China expert based in Boston who had advised Shen. China Thursday rejected a U.S. protest of the expulsion.

When the 1989 pro-democracy movement was crushed by the army, Shen fled to the United States and founded the Democracy for China Fund based in Newton, Mass. He wrote a book about the events, "Almost a Revolution."