HONG KONG (BLOOMBERG, REUTERS. AFP) - Several Hong Kong ex-lawmakers were among dozens taken into custody by authorities on suspicion of violating the city's national security law, in what appeared to be one of the most significant sweeps yet under the Beijing-drafted legislation.

Former lawmakers Alvin Yeung, James To, Andrew Wan, Lam Cheuk-ting were arrested on Wednesday morning (Jan 6) by the police's national security branch on allegations of subversion, according to Facebook postings.

The allegations were in relation to an informal primary held by opposition parties in July to choose candidates for a legislative election that was subsequently postponed by the government.

Some 50 people were swept up by police in the operation, local media reported. Activist Ventus Lau has also been arrested in relation to last year's legislative election primaries organised by the pro-democracy camp, a Lau associate said in a WhatsApp post.

The attempt to win a majority in the 70-seat city legislature, which some candidates said could be used to block government proposals and increase pressure for democratic reforms, was seen as an "act of subversion, in violation of the national security law", the party said.

The government had postponed the full election for the legislative council as it cited the coronavirus.

Among the 50 arrested on Wednesday was an American citizen working for a law firm, two sources told Agence France-Presse.

Mr John Clancey, a solicitor with the law firm Ho Tse Wai and Partners, was arrested on suspicion of "subversion".

A police source confirmed Mr Clancey's arrest and the charge.

Ho Tse Wai and Partners is known for taking on human rights cases and police were seen searching the firm's office on Wednesday morning.

One of its partners is Mr Albert Ho, a former Hong Kong lawmaker and opposition politician, who was also among some 50 opposition figures arrested in Wednesday's operation.

Mr Clancey is the first American national detained under the new security law in Hong Kong since its imposition in late June last year. He joined the company in 1997 and is a specialist in medical negligence and personal injury cases.

The law firm said he is also chairman of the Asian Human Rights Commission and a founding member of the Executive Committee of the China Human Rights Lawyers Concern Group.

New security law

The security law was imposed by Beijing on the former British colony in June.

It punishes what China broadly defines as secession, subversion, terrorism and collusion with foreign forces with up to life in jail and has been condemned by the West and human rights groups as a tool to crush dissent in the semi-autonomous, Chinese-ruled city.

Authorities in Hong Kong and Beijing say it is vital to plug gaping holes in national security defences exposed by months of sometimes violent anti-government and anti-China protests that rocked the global financial hub in 2019.

A police spokesman did not immediately respond to request for comment on Wednesday.

US President-elect Joe Biden's pick for Secretary of State Antony Blinken blasted Hong Kong authorities for the arrests.

"The sweeping arrests of pro-democracy demonstrators are an assault on those bravely advocating for universal rights," Mr Blinken wrote on his verified Twitter account.

"The Biden-Harris administration will stand with the people of Hong Kong and against Beijing's crackdown on democracy," he added.

Disqualification, exile

Since the imposition of the security law, leading pro-democracy activists such as media tycoon Jimmy Lai have been arrested, some democratic lawmakers have been disqualified, activists have fled into exile, and protest slogans and songs have been declared illegal.

Joshua Wong, 24, one of Hong Kong's most prominent democracy activists, was one of more than a dozen young, more confrontational politicians who outshone old guard democrats in the unofficial primary elections last July.

Wong's Twitter and Facebook accounts said his house was raided by police in the morning.

Wong was jailed last year on separate charges for organising and inciting an unlawful assembly during the 2019 anti government protests.

The success of young contenders in the democratic primaries, which Beijing said were illegal, came amid widespread resentment of a national security law that Beijing imposed last month.