ABOUT half a dozen dissidents detained in Shanghai are still being kept under police custody and the authorities have shown no signs of granting them an early release, sources said yesterday. Sources said both Yang Qinghen and Dai Xuezhong could face criminal charges but the authorities apparently still had not decided what to do with the other activists. Yang and Dai disappeared about three months ago and according to a note issued by the Public Security officials to Yang's family, the former editor of the unofficial journal Responsibility was taken by the police for an alleged criminal offence. Sources described Yang's charges as ''grave''. Dai, who was detained in early May by Shanghai police, was said to be under investigation for alleged tax problems, sources said. But they said the authorities were reluctant to reveal information to the families. Little was known about dissidents Li Guotao, Wang Fuchen, Bao Ge and Zhu Fuming and the authorities still denied they had taken these men into custody. The telephone line to Bao, who was active in seeking reparation for Chinese war victims from Japan, was cut after his arrest in June. When contacted earlier, Bao's family expressed concern that the 31-year-old medical researcher would be charged although the authorities have yet to reveal what law he has broken. Li, the chairman of the Association for Human Rights, reportedly disappeared in late April and has not reported for work since then. The authorities had declined to confirm whether they had arrested him. Yang Zhou, spokesman for the now-defunct unofficial Association for Human Rights, was still being held by police from Shanghai's Huangpu district. Sources said police had refused to release Yang because he had shown a ''bad attitude'' in detention. They said the police had told the family that they were unable to ''do work on him'' because of his attitude. They also said Yang, 50, was angered by the police's lengthy interrogation. It was reported that Shanghai police were keeping him under ''shelter and investigation'' although the detention order issued to his wife did not specify on what charges he was taken into custody. Sources also said they were less hopeful that Yang would be released soon since today marked the third month of his detention.