Hong Kong police have detained 47 pro-democracy activists on Sunday, accusing them of undermining national security laws introduced by China last year.

The activists were formally charged with conspiracy to commit subversion under the city’s national security law, in the largest mass charge against the semi-autonomous Chinese territory’s opposition camp since the legislation came into effect.

The former politicians and democracy advocates had been previously arrested in a sweeping police operation in January but were released.

They have been detained again and are scheduled to appear in court on Monday, police said in a statement.

Authorities allege they violated the national security law by participating in unofficial election primaries for Hong Kong’s legislature last year.

The move is part of a continuing crackdown on the city’s democracy movement, with a string of arrests and prosecutions of Hong Kong’s democracy proponents – including outspoken activists Joshua Wong and Jimmy Lai – following months of anti-government protests in 2019.

An hour after #HongKong authorities’ mass arrest of 47 pro-democracy activists, dozens of police officers pin a civilian to the ground during a booth held by students’ and medical workers’ unions.#SaveHongKong
Source: Guardians of Hong Kong pic.twitter.com/UqB7Yt0WIj

— Eileen Chang 😷⛑🥽🌂🧤🇭🇰 (@EileenEChang) February 28, 2021

The pro-democracy camp held the primaries to determine the best candidates to field to win a majority in the legislature.

It had plans to vote down major bills that would eventually force Hong Kong leader Carrie Lam to resign.

In January, 55 activists and former politicians were arrested for their roles in the primaries.

Authorities said the activists’ participation was part of a plan to paralyse the city’s legislature and subvert state power.

The legislative election that would have followed the unofficial primaries was postponed by a year by Ms Lam, who cited public health risks during the coronavirus pandemic.

Mass resignations and disqualifications of pro-democracy politicians have left the legislature largely a pro-Beijing body.

Former legislator and District Council member Gary Fan hands himself in to Hong Kong authorities. Photo: AP

Among those arrested on Sunday was former politician Eddie Chu. A post on his official Twitter account confirmed he was being charged for conspiracy to commit subversion and he had been denied bail.

“Thank you to the people of Hong Kong for giving me the opportunity to contribute to society in the past 15 years,” Mr Chu said in a post on his Facebook page.

Another candidate in the primaries, Winnie Yu, was also charged. She will appear in court on Monday, according to a post on her official Facebook page.

The security law criminalises acts of subversion, secession, terrorism and collusion with foreign powers to intervene in Hong Kong’s affairs.

Serious offenders could face a maximum punishment of life imprisonment.

Nearly 100 people have been arrested since the law was implemented.

-AP/ABC