A man who went berserk in a flat with a chopper, injuring four policemen, died yesterday soon after being overpowered by officers. One of the constables, a 36-year-old surnamed Li, had his right thumb severed. Doctors at Princess Margaret Hospital last night were trying to reattach it. He was in a serious condition. Police used pepper gas, batons and riot shields to subdue Cheung Chee-yung, 49, after they were called to a flat in Ho Pui Street, Tsuen Wan, at 4.05pm. Acting Tsuen Wan District Commander (Crime) Ma Kai-keung said Cheung had suffered no obvious injuries and the cause of his death was not immediately known. A postmortem examination will be carried out today. Mr Ma said Cheung had been conscious when taken from the scene by ambulance. But his face turned blue on his way to Yan Chai Hospital. He was declared dead at 5.22pm. Mr Ma said police resorted to using pepper gas on Cheung because he was 'very violent'. He said it took at least five officers with batons and shields to pin Cheung to the floor, and during the struggle two sergeants and two constables had been injured. Mr Ma said the officers from the Emergency Unit, New Territories South, responded to a 999 call from Cheung's father, who told police Cheung was 'chopping everything in sight' in their third-floor flat. Mr Ma said Cheung charged at the officers when they arrived. Reinforcements were called and with their help Cheung was overpowered and disarmed. The other three injured officers, including one with a slash wound to the back of his neck, were discharged after treatment at Yan Chai Hospital.