China has moved to crush Hong Kong's democracy movement in recent months but Wednesday's mass arrest of democracy figures for subversion under Beijing's new national security law was particularly momentous.

Here's why.

The most eye-catching element is the scale of the crackdown and the variety of figures caught in the dragnet.

Prior to Wednesday, around 30 people had been arrested under the new security law since its imposition in late June last year.

That figure was eclipsed in a single day with 53 simultaneous arrests carried out by more than 1,000 officers.

The list of those rounded up is a who's who of the democratic opposition, from elderly veterans and youth activists, to lawyers, academics and social workers.

In a first, it also includes an American national.

Bail is not usually granted for those charged with security crimes and offences can carry up to life in prison.

Subversion is one of the four new crimes outlawed by the security law.

The others are secession, terrorism and colluding with foreign forces.

Beijing says the law is needed to return stability to Hong Kong after huge democracy protests in 2019 and says other countries also have similar security laws.

But few contain the kind of catch-all wording China uses.

The definition of subversion, for example, includes any attempt to "seriously interfere in or disrupt" the government.

Wednesday's arrests reveal Hong Kong authorities have decided that an unofficial primary election organised last year by pro-democracy groups was subversion and therefore a national security crime.

A primary is a process in which voters cast ballots for their preferred candidates to compete in a future election.

In 2020, Hong Kong's pro-democracy coalition groups held an unofficial primary to choose their candidates for upcoming legislative elections.

Hong Kong police carried out mass arrests of opposition figures on Wednesday for subversion under the new security law
Hong Kong police carried out mass arrests of opposition figures on Wednesday for subversion under the new security law AFP / Peter PARKS

The 70-seat body is only half directly elected, a system that all but guarantees pro-government control.

But democracy supporters had been hoping to take all 35 electable seats by capitalising on swirling anger towards the city's pro-Beijing leaders after the protests of 2019.

After that, they could try to block legislation or even force a vote of no confidence in Chief Executive Carrie Lam.

In the end, the election was postponed for a year with authorities blaming the coronavirus.

But Beijing decided the campaign was nonetheless subversion.

Hong Kong and Beijing have defended the move.

The city's security minister, John Lee, said those arrested were trying to "sink Hong Kong into an abyss".

Beijing's Liaison Office said those facing prosecution "strategically organised or implemented a plan to paralyse the government".

Opposition figures are horrified.

They say the subversion charges prove that virtually any opposition to government policy is now deemed a security crime.

"Trying to stand in an election, publishing your political ideals, organising public gatherings and expressing your preference in an unofficial primary survey are all considered attempts to 'subvert the state's power'," the Hong Kong Democracy Council, a body largely made up of overseas exiles, said in a statement.

Amnesty International said the arrests "illustrate how the broad scope of the law allows it to be applied in circumstances that do not qualify as genuine threats to national security."

Hong Kong is a major international business hub.

So far most business organisations have stayed quiet about the crackdown, fearful of incurring Beijing's wrath.

Kristian Odebjer, chair of the Swedish Chamber of Commerce in Hong Kong, was a rare exception Wednesday.

"HK political crackdown is having a significant negative impact on business," he wrote on Twitter.

"Legitimate questions about rule of law raised and as HK image deteriorates, it becomes more and more difficult to defend why you should maintain costly operations here."