ABOUT 200 residents marched to the Tuen Mun police station yesterday to demand that more policemen patrol the district. Representative Cheung Yuet-lan, a Meeting Point Tuen Mun District Board member, said the recent rape and assault cases illustrated clearly that there were not enough police to cover the district. She said Tuen Mun had a population of more than 400,000 but only about 700 policemen. Yau Oi Estate, which housed 30,000 people, was covered by only two policemen, Ms Cheung said. ''What can we expect from this tiny police presence?'' she asked. Ms Cheung said security measures such as video cameras and metal gates were essential and should be installed in Tuen Mun public housing estates. She said residents would try to meet the chairman of the Housing Authority, Rosanna Wong Yick-ming, to press for an early start to installation work.