Hong Kong national security police on Thursday arrested 10 people for "collusion with foreign forces" and "inciting riot" over a now-defunct fund set up to help those targeted for involvement in the 2019 protest movement.

"The National Security Department of the Hong Kong Police Force today ... arrested four men and six women, aged between 26 and 43, in various districts for suspected 'conspiracy to collude with a foreign country or with external elements to endanger national security,' ... and inciting riot," the police said in a statement on the government's website.

"The arrested persons were suspected of conspiracy to collude with the 612 Humanitarian Relief Fund to receive donations from various overseas organizations to support people who have fled overseas or organizations which called for sanctions against Hong Kong," the statement said.

The arrests come after the arrests of Cardinal Joseph Zen and other trustees of the now-disbanded Fund prompted an international outcry in May 2022.

Police searched the arrestees' homes and offices with court warrants, seizing documents and electronic communication devices, it said, adding that the 10 are being held "for further enquiries."

"The possibility of further arrests is not ruled out," it said, warning the general public "not to defy" the national security law.

Hong Kong police typically don't name arrestees, but Reuters identified one of the 10 as pro-democracy activist Bobo Yip, who was photographed waving at journalists as she was taken away.

The London-based rights group Hong Kong Watch said the arrests were a "new low" in an ongoing crackdown on dissent under the national security law, which was imposed on the city by Beijing in the wake of the 2019 protests.

"Today’s arrests mark a new low in the deterioration of Hong Kong’s rights and freedoms," the group's research and policy advisor Anouk Wear said in a statement.

"It was already an overly broad and political interpretation of the law, including the National Security Law, to arrest and fine the trustees and secretary of the 612 Humanitarian Relief Fund last year," Wear said.

In May 2022, police arrested five former trustees of the fund – retired Catholic bishop and Cardinal Joseph Zen, ex-lawmakers Margaret Ng and Cyd Ho, Cantopop singer Denise Ho and cultural studies scholar Hui Po-keung – on suspicion of "conspiring to collude with foreign forces."

While they were never charged with the offense, the five were later found guilty of failing to register the fund – which offered financial, legal and psychological help to people arrested during the 2019 protest movement – and were each fined H.K.$4,000.

"The arrest of the 612 Humanitarian Relief Fund’s staff for alleged collusion and rioting is an absurd criminalization of providing legal and humanitarian aid," Wear said.

"This is an attempt by the Hong Kong government to rewrite history and frame all association with the protest movement as criminal, which is deeply damaging to rule of law and civil society."

Zen, whose passport had been confiscated following his arrest as a condition of his bail, was allowed to retrieve it to attend the funeral of Pope Benedict XVI in January, handing it back again on his return.

Zen was among six Hong Kongers nominated for the 2023 Nobel Peace Prize in February.