A HIGH Court judge condemned mainland criminals yesterday for treating Hongkong as a ''robbers' paradise'' and openly challenging its system by firing guns in crowded streets. Justice Chan, handing down prison terms of up to 28 years to three men involved in armed raids on jewellery shops in Central, said this type of crime had to be stopped. The robberies took place at the Chow Tai Fook goldsmiths in Queen's Road Central, on October 26, 1991, and March 12, 1992. On both occasions, the gang fired at police and hijacked cars to escape. The raids were also at busy times of the day, with Central streets filled with office workers and shoppers. During the first holdup, a police inspector was wounded and the robbers made off with $20 million of valuables. In the second raid, they took a security guard hostage and fled with gold ornaments worth $12 million. When some of the gang were stopped by police in Chai Wan a few hours after the second holdup they opened fire, hitting a woman in the knee. Mr Justice Chan said the attacks were horrifying. He was alarmed by the robbers' readiness to open fire on the crowded streets of Hongkong and their total disregard for human life. ''I am astonished at the relative ease with which they can lay their hands on lethal firearms. ''This must be put to a stop and deterrent sentences are necessary. If these activities are not stopped, they will become more prevalent in 1997 and beyond.'' Mr Justice Chan was also appalled by ''the almost unrestricted freedom with which they come in and out of Hongkong and escape the long arm of law''. Saying that the Chow Tai Fook raids were indicative of ''a type of joint venture'' between Chinese and Hongkong robbers, Mr Justice Chan sentenced mainland illegal immigrants Chau Sai-ming, 21, and Ng Yuk-ming, 30, to 28 and 25 years' jail respectively, for robbery and possession of firearms. Chau and Ng had earlier pleaded guilty to the offences, and while Ng had given evidence for the Crown against a third robber Tsang Tin-hei, 26, Chau had testified for the defence. Tsang, who was found guilty by a jury on one count of robbery and possession of a firearm, was sentenced to 20 years' jail. Mr Justice Chan agreed that Ng should be put in the category of ''supergrass'' for testifying against Tsang for his involvement in the March robbery. But he said the sentences were to serve as a stern warning that law-abiding citizens must be protected. Counsel for Ng, Michael Ko, submitted that his client had co-operated with the police from the start. It was based on his information that Tsang had been arrested. Christopher Young, for Chau, said that while his client was carrying a gun, there was no evidence to suggest that he had opened fire. Tsang's counsel, William Lee, said his client was not the mastermind of the crime and there was no evidence to show he had profited by the robbery.