CHINA yesterday detained its best-known dissident, Wei Jingsheng, less than six months after he was released from more than 14 years in jail. The detention came less than a week after US Assistant Secretary of State John Shattuck delivered a letter from a group of US Congressmen to President Jiang Zemin saying China would lose its Most Favoured Nations (MFN) trading status this June if more political prisoners were not released. It also came a week before the US Secretary of State Warren Christopher's visit to China. Mr Christopher is to tell Chinese leaders that billions of dollars worth of exports to the US are in jeopardy because of their human rights record. The detentions drew an immediate response from Mr Shattuck who left Shanghai yesterday saying he was ''disturbed'' to hear of the crackdown and again urged Beijing to free all political prisoners. Asia Watch, the human rights watchdog, last night issued a statement urging Mr Christopher to postpone the visit to China in view of the latest crackdown. The detention of Wei followed the arrest of at least four other dissidents in Beijing on Thursday and is being seen as part of a concerted effort to keep Beijing's dissident community quiet during the forthcoming session of the National People's Congress. Wei was taken away from his office at 10 am yesterday by three plain-clothes police officers and escorted to a police station for questioning, according to a friend contacted by Wei's secretary. ''His [Wei's] secretary said he would have to cancel our appointment as he would probably be away for a few days,'' he said. Former student leader, Wang Dan, was detained for 24 hours along with two friends at Beijing Railway Station on Wednesday as they were about to board a train to the central Chinese city of Zhengzhou. Mr Wang said yesterday the police gave no explanation as to why he was being held. ''It was very strange, they did not interrogate me, just kept me at the railway police station for a few hours,'' he said. Mr Wang, a key figure in the 1989 student demonstrations, said he did not believe the authorities would be able to keep Wei locked up for too long. ''They will probably detain him for a few days and try to persuade him to go out of town for the duration of the NPC,'' Mr Wang said, ''but knowing Wei Jingsheng, he will not go voluntarily.'' The Foreign Ministry refused to comment on the detention of Wang and Wei but confirmed the arrest on Thursday of three other dissidents, including Zhou Guoqiang, the Beijing-based lawyer of labour leader Han Dongfang. ''Zhou Guoqiang and Wang Jiaqi violated criminal laws and were arrested and investigated by the [Beijing] judicial authorities,'' the ministry said yesterday. A third dissident, Yuan Hongbing, ''took part in a criminal action and was arrested and investigated by the Guizhou judicial authorities [in southwest China]'', it said. In Shanghai, at least four activists were detained for 24 hours - apparently to stop them from meeting Mr Shattuck. Yang Zhou, a veteran activist there, said he was freed yesterday afternoon after being locked up at a police station. According to Yang, he was scheduled to meet Mr Shattuck on Thursday, but was unable to attend because of ''strong police surveillance''. ''I had planned to tell Mr Shattuck that the US should give China MFN this year. After all there has been progress on human rights in the country over the years,'' Yang said. ''But I am very angry over what they did. I have done nothing wrong and the detention is absolutely illegal,'' he added. Three others freed yesterday were medical teacher Bao Ge and activists Dai Xuezhong and Yang Qinheng.