A 90-year-old Roman Catholic archbishop, a musician, and at least two people have been arrested in Hong Kong on accusations of collaborating with foreign forces to jeopardize China's national security, in a move generally viewed as yet another example of Beijing's erosion of rights in the country.

The arrests are part of a broader campaign in the city on all types of protest that seems increasingly vengeful in punishing conduct taken before the national security law was passed. 

The crackdown is spreading to the city's long-respected commercial, religious, and educational institutions, as well as non-governmental groups, many of which have shut down activities in Hong Kong.

According to a police statement, two men and two women between the ages of 45 and 90 were arrested on Wednesday as commissioners of the 612 Humanitarian Relief Fund, which offered legal assistance to those who participated in the 2019 pro-democracy uprisings that were put down by security forces.

Another individual, a 37-year-old man, was charged with failure to properly register the fund, which was shut down in 2021. Those detained had been told they would have to give their travel documents and be freed on bail.

More arrests are expected in the case, according to the police statement, which did not name the individuals held.

Cardinal Joseph Zen, lawyer Margaret Ng, academic Hui Po-Keung, singer-actress Denise Ho, and former Legislative Council member Cyd Ho Sau-lan were among those participating, according to rights groups. 

It was unclear whether Hui had been officially detained. Zen was last seen exiting a police station at about 12 a.m. on Wednesday.

Beijing has recently imposed a comprehensive National Security Law on the city, and dozens of pro-democracy activists were detained, including senior politician Martin Lee and publisher Jimmy Lai.

The city's independent press has been demolished, and the legislature has been reconfigured to cram it with China loyalists.

Zen, the retired archbishop of Hong Kong, is a harsh critic of China, and he has been particularly harsh in his disapproval of the Vatican's 2018 accord with Beijing on bishop appointments, which he has described as a betrayal of China's covert Christians.

The Vatican's spokesperson, Matteo Bruni, said the Holy See "received with worry the news of Cardinal Zen's detention and is closely monitoring the situation's development."

Hui was detained at Hong Kong's international airport as he attempted to flee the city, according to Hong Kong Watch, a U.K.-based human rights organization.