The operation was sparked by an unofficial primary poll that democracy groups held last year to choose candidates for an ultimately scrapped local election.

At the time Beijing warned the primary was "subversion" because the opposition was attempting to win a majority in the partially elected legislature that would have allowed them to block government legislation.

Those detained on Wednesday represent a broad cross-section of the democracy movement.

Here are four notable figures who were arrested.

The American lawyer

American national John Clancey is a veteran lawyer at Ho Tse Wai and Partners, a firm known for taking up human rights cases.

He is the first US national detained under the new security law.

Fluent in Cantonese, he has been practising in Hong Kong since 1997 and has been a key figure within legal groups advocating for greater democracy and human rights protections.

He is the chairman of the Asian Human Rights Commission and the Asian Legal Resources Centre, as well as a founding member of the China Human Rights Lawyers Concern Group.

During the pro-democratic camp's primary election Clancey served as the treasurer of the Power of Democracy organising group.

The academic

Benny Tai, 56, is no stranger to arrest.

The devout Christian law professor has previously been jailed for helping to lead peaceful democracy protests in 2014 and last year lost his university job because of that conviction.

A non-violence advocate, he has embraced civil disobedience and was a key thinker behind last year's primary.

His idea was to unite Hong Kong's disparate democracy groups into a single coalition that could win a majority for the first time.

Only half the legislature's 70 seats are elected -- a system that all but guarantees loyalist control -- but if the opposition won all 35 seats it could block legislation.

At the time the plan was conceived, it was entirely legal, but halfway through the campaign Beijing's new security law was imposed and the primary was declared an illegal attempt to subvert the government.

The racial justice advocate

Hong Kong's South Asian and Black minorities have long complained of discrimination.

Jeffrey Andrews, a Hong Konger of Indian descent, is one of the few figures to take up the cause of racial justice, and the city's first registered ethnic minority social worker.

Last year the 33-year-old became more involved in politics, vowing to run for a seat in the legislative elections.

He took part in the primary, campaigning to give a voice to the city's minorities.

The former journalist

Gwyneth Ho, 30, became a hero to the democracy movement for her hours of live Facebook reporting of 2019's huge protests for local outlet Stand News.

She captured footage of a mob of government supporters attacking democracy activists at a train station, broadcasting even as the assailants turned on her.

The footage was hugely damaging to the police who were accused of taking to long to respond, allegations the force denies.

She announced plans to stand for election and took part in the primary but was among a dozen candidates disqualified by election officials because of their political views.