HONG KONG:Hong Kong police arrested 53 people in raids on democracy activists on Wednesday, with authorities saying an unofficial vote to choose opposition candidates in city elections was part of a plan to "overthrow" the government.

Dawn raids on 72 premises saw many of Hong Kong's most prominent pro-democracy advocates arrested in the biggest crackdown since Beijing imposed a new, controversial security law in the former British colony in June 2020.

The mass arrests were linked to an unprecedented, independently organised vote last July to select opposition candidates for a since-postponed legislative election.

About 1,000 police took part in the raids, which also included searches of the offices of a pollster and a law firm and went to the offices of media outlets Apple Daily, Stand News, and Inmediahk, according to media.

Secretary for Security John Lee said the group planned to cause "serious damage" to society and that authorities would not tolerate any subversive acts.

"The operation today targets the active elements who are suspected to be involved in the crime of overthrowing, or interfering seriously to destroy the Hong Kong government’s legal execution of duties," Lee told reporters.

The arrests, confirmed by the Democratic Party and individual social media accounts of dozens of other activists and politicians arrested, will further raise alarm that Hong Kong has taken a swift authoritarian turn.

The crackdown since the June 2020 imposition of the new security law, which critics say crushes wide-ranging freedoms promised when the city returned to Chinese rule in 1997, places China further on a collision course with the United States just as Joe Biden prepares to take over the presidency.

The White House did not immediately respond to a request for comment. Biden's pick for secretary of state, Antony Blinken, said on Twitter the arrests were "an assault on those bravely advocating for universal rights".

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

The more than 50 arrests included former lawmakers, activists and people involved in organising the 2020 primary, among them James To, Lam Cheuk-ting, Benny Tai and Lester Shum.

At the time, the city government and Beijing warned the unofficial democratic primary may violate the new law, saying a campaign to win a majority in Hong Kong's 70-seat legislature with the purpose of blocking government proposals to increase pressure for democratic reforms could be seen as subversive.

The city's leader is not directly elected and only half the legislative seats are, with the other half stacked mostly with pro-Beijing figures.

"The people involved are suspected of making use of what they call a '35+' plan ... to somehow paralyse the Hong Kong government," Lee said, adding the government would not tolerate any "subversive acts."

Police did not immediately comment.

The legislative election was due in September last year but was postponed, with authorities citing coronavirus risks. It is unclear who could run for the opposition in any future polls following the mass arrests.

Maya Wang, senior China researcher at Human Rights Watch, said the raids and arrests showed Chinese authorities were now "removing the remaining veneer of democracy in the city".

Media said the police operation included searches of the offices of the Hong Kong Public Opinion Research Institute (HKPORI) which helped organise the primaries. The organisers destroyed the data of the more than 600,000 people who voted immediately after ending the count.

Public broadcaster RTHK said American lawyer John Clancey was arrested during a raid of law firm Ho, Tse, Wai & Partners.

"PURGATORY OF THE ORIENT"

The security law punishes what China broadly defines as secession, subversion, terrorism and collusion with foreign forces with up to life in jail. When the law was introduced, authorities said it would only target a very small group of people in the city of 7.5 million.

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.

In response to the law, Washington imposed sanctions on Hong Kong and Beijing officials and several countries suspended extradition treaties.

Since its imposition, 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.

Taiwan's government said it was "shocked" at the arrests, adding Hong Kong had changed from the "pearl of the Orient" to "purgatory of the Orient".

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 the old guard in the unofficial democratic primaries in 2020.

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

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