Monday, June 21, 1999; 6:48 a.m. EDT

BEIJING (AP) -- An exiled Chinese dissident who disappeared six months ago after sneaking into China is in jail and in bad health, according to a letter smuggled out of prison and released by dissident groups today.

In the letter to his wife, Zhou Yongjun said he was sentenced to three years in prison -- the first word on his fate since he called her in New York on Dec. 21 to say his arrest was imminent.

The letter, released by the Washington-based Free China Movement and the New York-based Chinese Democracy, and Justice Party did not say why he had been jailed.

Zhou slipped into China after being turned back on Dec. 16 by border police at Shenzhen, the boomtown that abuts the open city of Hong Kong.

A leading figure in the 1989 democracy movement, Zhou captured world attention by kneeling on the steps of the Great Hall of the People above Tiananmen Square to humbly petition China's communist leaders to acknowledge the student demonstrators.

After serving two years in prison, he moved to New York. He decided to return to China to visit his elderly parents and to sneak in a manuscript about a dissident poet he had hoped to publish.

Chinese authorities have for years kept a ``blacklist'' of overseas activists and foreigners they want kept out of China.

``I am very sorry and regretful for having left behind the days of family togetherness in the United States with you and our son,'' Zhou wrote to his wife in the letter from the Huanghua detention center in Guangzhou, a southeastern provincial capital.

``You must have the courage to face a bitter life ... Fate is like this,'' said the letter, dated April 15.

Zhou said he has twice suffered from high fevers and was constantly taking medicine. ``I feel my health going from bad to worse. My spirits are not buoyant'' Zhou wrote.

A group of democracy advocates petitioned the national police force Sunday to release 30 dissidents taken into custody in the weeks before June 4, the 10th anniversary of the military assault that ended the Tiananmen Square protests.

Separately, two members of an illegal opposition party were detained Sunday, the Hong Kong-based Information Center of Human Rights and Democratic Movement in China reported.

Police took Li Xi'an and Chi Jianwei of the China Democracy Party away from a meeting of dissidents in the eastern city of Hangzhou, near Shanghai, the Information Center said. Twelve other Democracy Party members in Hangzhou are also in detention, the group said.