China has arrested four more dissidents, bringing the number of pro-democracy activists detained since US President Bill Clinton's visit last month to 21, dissident sources said. The Information Centre of Human Rights and Democratic Movement in China said the four dissidents were arrested on Friday evening in the town of Xiantan in the southern province of Hunan.

The Hong Kong-based group said five policemen raided the house of dissident Jin Jiwu, where they also picked up Liu Hai, Li Yingzhi and Song Ge. The group said the police searched Jin Jiwu's house and confiscated a computer, an address book and some books. It said the four dissidents had met to discuss the detention of their colleague Zhang Shanguang, arrested four days ago, and to write an open letter calling for his release.

Zhang was arrested while attempting to set up a union to defend the rights of workers laid off by the public sector. Jin Jiwu, 36, has already spent a year in detention for his part in the 1989pro-democracy demonstrations in China. Another pro-democracy campaigner, Fan Yiping, was sentenced this week to three years in a labour camp.

The crackdown pushed 101 dissidents from the usually splintered and disparate Chinese pro-democracy groups in the United States to write an open letter of protest. ``We strongly protest the Chinese government's behaviour in trampling human rights, and support the just struggle of the Chinese Democracy Party,'' said the dissidents, who included the father of China's pro-democracy movement Wei Jingsheng.