The United Nations voiced alarm Thursday at the arrest of 53 prominent figures in Hong Kong on charges of "subversion", urging their immediate release.

In an operation involving 1,000 officers, Hong Kong police arrested the activists for "subversion", a new national security crime that carries up to life in prison.

"We are deeply concerned about the arrests on Wednesday of 53 political activists, academics, former legislators, current district councillors, and lawyers in the Hong Kong Special Administrative Region, and we call for their immediate release," UN rights office spokeswoman Liz Throssel said in a statement.

"Yesterday's arrests were the latest in a series of detentions related to the exercise of fundamental freedoms, including the right to peaceful assembly, in Hong Kong," she said.

Wednesday's arrests under a new security law marked the latest salvo in Beijing's battle to stamp out dissent in the semi-autonomous city after millions hit the streets in 2019 with huge and sometimes violent democracy protests.

"These latest arrests indicate that - as had been feared - the offence of subversion under the National Security Law is indeed being used to detain individuals for exercising legitimate rights to participate in political and public life," Throssel said.

She pointed out that the UN rights office and rights experts had repeatedly warned that offences like subversion under the new law, passed last June, were "vague and overly broad, facilitating abusive or arbitrary implementation."

"We stress that exercise of the right to take part in the conduct of public affairs, directly and through freely chosen representatives, is a fundamental right" under international law and Hong Kong's Basic Law.

The UN rights office, she said, called on authorities to "refrain from using the National Security Law to suppress the rights to freedom of expression, peaceful assembly and association."

"We also urge the authorities to guarantee the right to freedom of expression in the context of ongoing investigations, including by allowing journalists and news organisations to fully and freely exercise their legitimate functions."

nl/apo/wdb