HONG KONG — Hong Kong’s high court on Thursday sentenced a man to 23 years and 10 months in prison for being the mastermind of an alleged plot to attack police with explosives during the city’s pro-democracy and anti-China protests in 2019.

The man, Ng Chi-hung, 28, had been charged with “prohibitions against bombing of prescribed objects” under the United Nations Anti-Terrorism Ordinance and a conspiracy to use explosives and firearms to endanger life.

Another defendant, Wong…