A pro-democracy group that organized some of the biggest protests during months of political upheaval in Hong Kong in 2019 is dissolving, the group said Sunday.

The Hong Kong Civil Human Rights Front, made up of a slew of member organizations, said it could no longer operate. The decision comes as the group faces a police investigation for possible violation of a national security law, according to local media.

The group, which also organized an annual protest march marking the semiautonomous territory’s handover to China in 1997, is the largest to disband amid a sweeping crackdown on dissent in the city. Earlier this week, the city’s largest teachers’ union disbanded in light of what it said were drastic changes in the political environment.

Rights group Amnesty International expressed its concern over “the pattern of self-censorship seen this week." The London-based organization said in a statement this “signals a concerning domino effect, as Hong Kong’s draconian national security law has triggered an accelerating disappearance of independent civil society groups from the city.”

The crackdown follows Beijing’s imposition of the national security law on Hong Kong last year. The legislation outlaws secession, subversion, terrorism and foreign collusion and has been used to arrest more than 100 pro-democracy figures since it was first implemented a year ago, as well as the closure of pro-democracy newspaper Apple Daily.