More than 50 Hong Kong pro-democracy activists have been arrested on suspicion of violating the city’s national security law in the biggest crackdown yet against the opposition camp under the contentious new legislation.

Police also arrived at the offices of pro-democracy online media outlet Stand News, according to live footage on its website on Wednesday.

A Stand News reporter said police had asked the editor-in-chief to sign documents related to a national security investigation.

She said the media group would consult lawyers.

• Read: China’s parliament passes Hong Kong security law

The dawn sweep of some of the city’s most prominent activists and former democratic MPs will further raise alarm that Hong Kong has taken a swift authoritarian turn.

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

The arrests on Wednesday included former MPs and activists James To, Lam Cheuk-ting and Lester Shum, according to the Democratic Party’s Facebook page and public broadcaster RTHK.

Police did not immediately respond to requests for comment.

The Democratic Party’s Facebook page said police arrested the activists for participating in an independently organised, unofficial ballot in July 2020 to select democratic candidates for a legislature election, which the Hong Kong government and Beijing warned at the time may violate the new law.

The legislative election was due in September last year but postponed, with authorities citing coronavirus risks.

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.

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”.

“Beijing once again has failed to learn from its mistakes in Hong Kong: that repression generates resistance, and that millions of Hong Kong people will persist in their struggle for their right to vote and run for office in a democratically elected government,” Wang said.

The security law 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 former British colony.

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.

-AAP