POLICE yesterday detained a 27-year-old man in connection with two sexual assaults in Tuen Mun last week - but they have ruled out any link between him and the rape and murder of 19-year-old Ip Wing-sze. At the same time, residents of the beleaguered Yau Oi estate, where one of the attacks took place, have decided to set up vigilante groups to protect women from further assaults. The man was detained after he was found loitering in the lobby of Leung Shui House. He allegedly matched the description of a man thought to be responsible for sexual attacks at Tai Hing estate last Tuesday and at Yau Oi estate on Thursday. His flat was searched and last night he remained in custody, although no charges have been laid. Authorities have stepped up patrols in Tuen Mun since last Sunday's brutal murder, but residents at Yau Oi estate, claiming the police were still not doing enough to ensure their safety, decided last night to set up their own ''civil patrols''. Officers strongly denied they were not doing enough and warned the vigilantes could be tempted to act illegally if they caught someone they believed had done wrong. However, the move was supported by the Housing Authority and representatives of the City and New Territories Administration. Residents are now planning a referendum of fellow tenants to decide how the vigilante groups should be established. Police officers have carried out thousands of interviews around Leung King estate during the past week. Chief Inspector Wong Wing-kee said more than 40 officers were involved in door-to-door interviews covering 2,800 flats. Many neighbours had provided helpful information, ''but it's too early to say whether these will actually turn out to be leads'', he said. Meanwhile, sales of personal alarms and self-defence weapons have boomed since the murder. ''More than 20 personal emergency alarms were sold just two days after the tragedy. All the buyers were from Tuen Mun,'' said Bonnie Mok Ching-yee, secretary of the firm, National Fire and Security South East Asia. Miss Mok said the phenomenon was not new; the business boomed whenever rapists were active. ''I remember when there were serial killings and rapes in other estates in Tuen Mun last year. We were flooded with orders. Then it stopped when police said they'd arrested the wanted person.'' Most customers wanted pager-size alarms and perfume-sized sprays, she explained. She said the liquid in the spray was made mainly from chillis and was used by the Agriculture and Fisheries Department and the Environmental Protection Department to deal with wild dogs. ''If you spray it on somebody's face, it will blind them for some time,'' she said. Security firm Hong Kong Guards, which serves 11 housing estates in Tuen Mun for the Housing Authority, declined to reveal details of its business apart from saying it had hired an additional 20 people since the attacks. ''They're employed to sit by the waiting posts or to act as escorts. We do not expect them to act like the police and catch the rapist,'' said a spokesman.