Security at the Jewellery and Watch Fair has been stepped up in a bid to prevent a repeat of thefts which have marred past exhibitions. Asia's biggest jewellery fair opened at the Convention and Exhibition Centre yesterday with more than 1,500 exhibitors expecting 30,000 visitors during the five-day show. In previous years the exhibition has been marred by theft of valuable items, including a $2.3 million diamond in 1994. But police and security staff hired by the organisers, Miller Freeman Asia, have increased surveillance to prevent similar incidents. Armed guards and closed-circuit television will survey the scene and exhibitors have been briefed about security. Crime Prevention Bureau Chief Inspector Robert Ho Kwok-fai said police were 'very cautious' about the exhibition in the Wan Chai centre. 'Police deployment has been strengthened because there are a lot of valuable items,' he said. Policing in the vicinity of the exhibition has been stepped up, although Wan Chai's assistant director of operations, Michael Chiu Kai-ting, would not say how many officers were deployed. Normally, police do not patrol private premises and the force refused to say whether plain-clothes officers would survey the exhibition hall.