About 50 pro-democracy lawmakers and activists have reportedly been arrested in Hong Kong under the controversial national security law, reports BBC.
The Democratic Party's Facebook page said the arrests were related to an independently organised primary vote last year to select democratic candidates for a legislature election.
Leader Carrie Lam had warned at the time it would amount to subversion.
Beijing imposed the law on the former British colony in June last year.
The wave of arrests is the biggest crackdown on Hong Kong's pro-democracy opposition under the new legislation.
It is thought to include well-known opposition figures from both the Democratic Party and the Civic Party like James To, Lam Cheuk Ting, and Lester Shum.
An alliance of opposition parties ran independently organised primaries in July to determine which of their candidates would have the best chances in September's election for the Legislative Council, Hong Kong's parliament. 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.
What is Hong Kong's security law?
The security law punishes what China broadly defines as secession, subversion, terrorism and collusion with foreign forces with up to life in jail.
It has been widely criticised by rights groups and Western nations as effectively curtailing dissent.
The Chinese government has defended the law, saying it will help return stability to the territory, which has been shaken by pro-democracy protests, and bring it more into line with the Chinese mainland.
After it law was introduced, a number of pro-democracy groups disbanded out of fears for their safety.
Over the past weeks and months, several high profile court cases under the security law have gotten underway.
Media tycoon Jimmy Lai has been charged under it as well as several activists who tried to flee the territory by boat last August.