47 Hong Kong opposition activists were arrested and faced charges of "subversive conspiracy" under national security law.
Among these is activist Sam Cheung, 27, who participated in the primary election held by the Democratic Party last July.
Cheung was arrested with 54 other activists on January 6 during the largest police raid and mass arrest campaign implemented since the national security law came into effect in June last year.
The Democrats hold primaries in the five electoral districts to determine who will run in the Legco's elections in September 2020.
The "35 plus" strategy, if successful, would allow the party to form a powerful bloc in Legco to effectively block budgets and bills and paralyze the government.
According to Hong Kong police, 47 opposition activists were charged with the same crime and the trial will take place tomorrow morning.
Of those arrested in January, only eight have not been charged, including US human rights lawyer John Clancey and veteran activist James To, who are still on bail.
Hong Kong security law criminalizes four types of national security crimes, including separatism, subversion, terrorism, and collusion with foreign or outside parties to endanger security.