HONG KONG (AP) — Hong Kong police arrested about 50 pro-democracy figures Wednesday for allegedly violating the new national security law by participating in an unofficial primary election last year held to increase their chances of controlling the legislature, according to political parties and local media.

Those arrested on suspicion of subversion included former lawmakers and pro-democracy activists, the South China Morning Post and online platform Now News reported.

The mass arrests were the largest move against Hong Kong’s democracy movement since the national security law was imposed by Beijing in the semi-autonomous territory in June last year. Police did not immediately comment on the arrests.

At least seven members of Hong Kong’s Democratic Party — the city’s largest opposition party — were arrested, including former party chairman Wu Chi-wai. Former lawmakers Helena Wong, Lam Cheuk-ting, and James To were also arrested, according to a post on the party’s Facebook page.

Benny Tai, a key figure in Hong Kong's 2014 Occupy Central protests and a former law professor, was also arrested by the police, according to local media reports. Tai was one of the main organizers of the primaries.

The home of Joshua Wong, a prominent pro-democracy activist who is serving a 13 1/2-month prison sentence for organizing and participating in an unauthorized protest last year, was also raided, according to a tweet posted from Wong's account.

According to tallies based on local media reports of the arrests, all the pro-democracy candidates who had participated in the unofficial primaries were arrested.

Police also went to the headquarters of Stand News, a prominent pro-democracy online news site in Hong Kong, with a court order to hand over documents to assist in an investigation related to the national security law, according to a livestreamed video by Stand News. No arrests were made.

In recent months, Hong Kong has already jailed several pro-democracy activists including Wong and Agnes Chow for their involvement in antigovernment protests, and others have been charged under the national security law including media tycoon and outspoken pro-democracy activist Jimmy Lai.

The security law criminalizes acts of subversion, secession, terrorism and collusion with foreign powers to intervene in the city’s affairs. Serious offenders could face up a maximum punishment of life imprisonment.

Pro-democracy activists and lawmakers had last July held an unofficial primary election to figure out which candidates they should field in a now-postponed legislative election that would boost their chances of gaining a majority of seats in legislature. Gaining a majority would allow the pro-democracy camp to vote against bills they deemed to be pro-Beijing, block budgets and paralyze the government.

More than 600,000 Hong Kongers voted in the primaries, although pro-Beijing lawmakers and politicians criticized the event and warned that it could breach the security law, which was imposed on the city by Beijing to quash dissent following months of antigovernment protests.

Hong Kong chief executive Carrie Lam said in July last year that if the primary election was aimed at resisting every policy initiative by the Hong Kong government, that it may fall under subverting state power, an offence under the national security law.

Beijing also blasted the primaries as illegal, calling it a “serious provocation” of Hong Kong’s electoral system.

Following the handover of Hong Kong to China by the British in 1997, the semi-autonomous Chinese city has operated on a “one country, two systems” framework that affords it freedoms not found on the mainland. In recent years, Beijing has asserted more control over the city, drawing criticism that Hong Kong’s freedoms were under attack.

The legislative elections, originally slated to be held in September, were later postponed for a year after Lam said that holding elections would be a risk to public health given the coronavirus pandemic. The pro-democracy camp denounced the postponement as unconstitutional.

In November, all of Hong Kong’s pro-democracy lawmakers resigned en masse after Beijing passed a resolution that led to the disqualification of four of its camp, leaving a largely pro-Beijing legislature.

“Beijing once again has failed to learn from its mistakes in Hong Kong: that repression generates resistance, and that millions of Hong Kong people will persist in their struggle for their right to vote and run for office in a democratically elected government,” Human Rights Watch’s senior China researcher Maya Wang said in a statement on Wednesday’s arrests.

In further remarks to The Associated Press, Wang said it wasn’t clear what provisions of the law were being cited to justify the arrests, but that local authorities seem less concerned with legal substance.

“The very nature of the national security law is as a draconian blanket law allowing the government to arrest and potentially imprison people for long terms for exercising their constitutionally protected rights,” Wang said.

“The veneer of rule of law is also applied in mainland China stripped of any meaning. Hong Kong is looking more like mainland China but where one ends and the other begins is hard to discern,” she said.