About 50 pro-democracy activists and politicians have reportedly been arrested in Hong Kong's biggest crackdown since the introduction of a controversial national security law.

The arrests are said to be linked to an independently organised primary vote.

Officers are also understood to have searched the house of the detained democracy activist Joshua Wong, raided a law firm and pressed news outlets to hand over information.

It is unclear if the action is related.

Police have yet to comment on the various moves officially.

The Democratic Party's Facebook page said the arrests were carried out under the security law imposed by Beijing on the territory last June in response to months of pro-democracy protests.

Among those detained are thought to be well-known opposition figures like James To, Lam Cheuk Ting, Lester Shum and Benny Tai, one of the initiators of the primaries.

According to local media reports, a US citizen working in Hong Kong as a lawyer on human rights cases has also been detained.

What were the July primaries?

In July, an alliance of opposition parties ran independently organised primaries to see which of their candidates had the best chances in September's election for the Legislative Council, Hong Kong's parliament.

More than 600,000 people voted in the primaries. The September election was later postponed, with officials citing concerns over the pandemic as the reason for the delay.

The opposition groups had hoped that winning more seats would give them enough power to block government proposals and increase pressure for democratic reforms.

Hong Kong's leader Carrie Lam had warned at the time the primaries could amount to subversion.

Several of the opposition figures planning to stand in the LegCo election have since been disqualified from running, including Joshua Wong and Lester Shum.

Former journalist Gwyneth Ho, who rose to prominence covering the 2019 protests and was later disqualified from running for LegCo, was also among those arrested on Wednesday.

Most of the opposition lawmakers in the current LegCo resigned in November en masse in protest to four of them being dismissed from parliament.

Aside from the string of arrests and the raid of Joshua Wong's home, police have also asked media outlets Stand News, Apple Daily and In-Media to hand over documents related to the national security law.