HONG KONG -- Chinese authorities may have arrested a U.S.-based dissident who sneaked into the mainland three weeks ago, a Hong Kong-based human rights group said on Monday.

Zhou Yongjun, a leader of the ill-fated 1989 pro-democracy movement, slipped into the mainland from Hong Kong on December 21 after being refused legal entry, said the Information Center of Human Rights & Democratic Movement in China.

"Student leader of the 1989 pro-democracy movement in Beijing Zhou Yongjun, who sneaked into China from Hong Kong on December 21, has lost contact with his families and friends for the past three weeks," it said. "He is expected to be arrested by Chinese police."

Zhou, a U.S. green card holder, had a valid travel document and an entry permit issued by the Chinese consulate in New York. But immigration officials in Shenzhen, southern China, refused Zhou entry on December 17 after questioning him for 10 hours, the center said.

It said Zhou, 31, then sneaked into the mainland to sign a business contract.

Zhou was jailed for two years for his role in the 1989 pro-democracy demonstrations. He fled to Hong Kong after being released from jail in 1992 and went to the United States in 1993, the human rights group said.