HONG KONG—The most high-profile case of Hong Kong’s yearslong effort to snuff out dissent after protests rocked the city began on Monday as newspaper tycoon Jimmy Lai went on trial under the national security law imposed by Beijing.

Lai, a child immigrant to the city who built a clothing and media empire, was among the most defiant critics of China’s Communist Party and a prominent backer of 2019 pro-democracy demonstrations through his now-closed Apple Daily.