Tony Chung, a former leader of pro-independence group Studentlocalism, was denied bail after appearing in West Kowloon Court on Thursday charged with secession, money laundering and conspiracy to publish seditious material.
The 19-year-old is the second person to be charged under Hong Kong's national security law.
The teenager is accused of organising, planning, committing or participating in acts with other people with a view to commit secession between July 1 and October 27 – the day of his arrest.
Under the national security law, people convicted of secession face a maximum punishment of life in prison.
Chung is also accused of conspiring to publish seditious material from November 30, 2018 to June 9 this year, with this offence covered by older legislation in the SAR.
Chung faces two money laundering charges, with prosecutors alleging he had funds totalling more than HK$600,000 in his PayPal and HSBC accounts.
No plea was taken, and chief magistrate Victor So adjourned the case to January 7, ordered that Chung be kept in custody in the meantime.
Article 42 of the national security law states that bail shall not be granted unless the magistrate has sufficient grounds for believing that the defendant "will not continue to commit acts endangering national security".
Prosecutors also said the police would need more time to check Chung's phone, laptop, bank records and any comments he had made online.
Two other former members of Studentlocalism were also arrested by the police on Tuesday, but were later released on bail.
When the national security law was imposed at the end of June, Studentlocalism announced that it would cease to operate in Hong Kong, although its overseas members would continue their activities.
The first person charged under the national security law was Tong Ying-kit, 23, who is accused of inciting secession and terrorism. He allegedly drove a motorbike into a group of police officers while carrying a "Liberate Hong Kong" flag on July 1, just hours after the new legislation came into force.
Tong was also denied bail and his legal team lost a court challenge over the decision.
Tony Chung denied bail after security law charge
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