SHENZHEN, China—U.S. tech giants and Chinese state-backed companies showed off the future of policing in this southern technology hub as they vie for a slice of the world’s biggest surveillance market.

Companies from across the globe packed one of the world’s biggest surveillance trade shows to demonstrate the latest gizmos and algorithms powering the high-tech revolution in the industry, of which China is on the vanguard. Tools being hawked included facial-recognition cameras, iris scanners, software that can read a subject’s...