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 Chun-keung, the leader of a group known as the “Dragon Slayers” that was active during the 2019 protests, was sentenced to 13 years and six months imprisonment.

Justice Judianna Barnes also sentenced five others to between 5 years and 10 months and 12 years for various offenses including possessing firearms and abetting the manufacture of explosives.

The long jail terms come amid a protracted national security clampdown by China in the global financial hub, and is the first time the U.N. anti-terrorism ordinance has been used in Hong Kong since 2002.

The judge told the court that the plan was premeditated, vicious and intended to target police officers in what she described as a “declaration of war on society.”

Steve Li, a chief superintendent for national security in the Hong Kong police, told reporters that the sentence was the heaviest for any offense linked to the 2019 protests so far, and that he believed it would have “a substantial deterrent effect.”

He noted that items had been seized including explosives and bullets, with the defendants accused of planning to plant two bombs during a protest march on Dec. 8, 2019, but some arrests were made and the plan didn’t go ahead.

One of the defendants, Lai Chun-pong, who had pleaded not guilty to the charges but was found guilty by a jury in August, was jailed for 10 years and 10 months. He was convicted of an alternative charge of conspiring to cause explosions that were likely to endanger life or cause significant property damage.

The other defendants had pleaded guilty.

The ruling came less than a week before 45 of Hong Kong’s leading democrats face sentencing in a separate national security case for an alleged subversive plot from 2020.