The Hong Kong police arrested 53 elected pro-democracy officials and activists early Wednesday for their involvement in an informal primary election, the largest roundup yet under the new national security law imposed by Beijing to quash dissent.

The mass arrests — which included figures who had called for aggressive confrontation with the authorities as well as those who had supported more moderate tactics — underscored Hong Kong officials’ efforts to weaken any meaningful opposition in the city’s political institutions. The police also visited the offices of at least one law firm and three news media organizations to demand documents, broadening the burst of arrests that started before sunrise and sent a chill through Hong Kong’s already-demoralized opposition camp.

The moves suggested that the authorities were casting a wide net for anyone who had played a prominent role in opposing the government. The national security law, which the Chinese government imposed in June, has been wielded as a powerful tool to crack down on the fierce anti-Beijing protests that upended the city for months. Since then, the Hong Kong authorities have detained pro-democracy leaders, raided news media offices and ousted opposition lawmakers.