Hong Kong — Hundreds of protesters flooded a Hong Kong court to support the appeals case of a jailed activist who played a key role in building a radical anti-China movement in the former British colony.

The crowds gathered outside Hong Kong’s high court in Admiralty, where self-described “localist” Edward Leung, 28, was expected to appear at an appeal hearing on Wednesday. Leung had been sentenced to six years in prison last year for helping to lead a 2016 riot that was one of the worst outbreaks of unrest the city had witnessed at the time...