Hong Kong police have arrested 53 people in raids on democracy activists, 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 controversial security law in the former British colony in June 2020.
The mass arrests were linked to an independently organised, and non-binding vote last July to select opposition candidates for a since-postponed legislative election.
About 1,000 police took part in the raids.
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 said.
The arrests will further raise alarm that Hong Kong has taken a swift authoritarian turn.
The crackdown - which is based on the June 2020 security law, which critics say crushes wide-ranging freedoms promised when the city returned to Chinese rule from the British in 1997 - places China further on a collision course with the United States as President-elect Joe prepares to take office on January 20.
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.
Those arrested included former politicians, activists and people involved in organising the 2020 primary, among them James To, Lam Cheuk-ting, Benny Tai and Lester Shum.
Police said campaigning 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 people involved are suspected of making use of what they call a '35+' plan ... to somehow paralyse the Hong Kong government," Lee said.
The legislative election was due in September last year but was postponed, with authorities citing coronavirus risks.
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".
American lawyer John Clancey was arrested during a raid of law firm Ho, Tse, Wai & Partners, a source at the firm said.
Visits to four media outlets, including anti-government tabloid Apple Daily, were about "transactions" with pro-democracy groups, and unrelated to reporting matters, police said.
The security law punishes what China broadly defines as secession, subversion, terrorism and collusion with foreign forces with up to life in jail.
The Hong Kong and Beijing governments 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.