Pro-democracy activists Joshua Wong, Agnes Chow and Ivan Lam were remanded in custody on Monday after pleading guilty to charges related to a major protest outside police headquarters last year.
The trio are expected to be sentenced on December 2.
"Everyone hang in there. Add oil!" Wong said before being escorted from the courtroom, while Lam raised his hand in a gesture signalling the "five demands" of anti-government protesters. Chow had no visible reaction to the magistrate's decision.
As supporters waited outside West Kowloon Magistracy for the prison van to emerge, they chanted "Release Joshua Wong! Release Agnes Chow! Release Ivan Lam!" as well as "Five demands, not one less".
Thousands of protesters laid siege to the police headquarters in Wan Chai on June 21 last year, to press the government to withdraw the later-abandoned extradition bill, and demanding the authorities stop categorising earlier protests as riots.
There were no major clashes with the police during the rally, but protesters blocked roads and the building's entrances, smashed surveillance cameras and vandalised the building's walls.
Wong, Chow and Lam – former leaders of the now-disbanded Demosisto group – admitted inciting people to join an unauthorised assembly.
Chow, 23, also pleaded guilty to taking part in the assembly, while Wong, 24, admitted organising the protest. They face a maximum punishment of three years behind bars.
The defence lawyers appealed for lenient sentences, urging the court to take into account the trio's young ages, Chow's clear record, and the fact that they did not incite or get involved in any violence at the protest.
Magistrate Wong Sze-lai reserved sentencing to next Wednesday, but ruled out community service for Lam, 26, due to his previous convictions.
While Wong and Lam have been in prison before, this is the first time Chow has been held in detention, although she did spend about 24 hours in police custody in August after being arrested under the national security law on suspicion of "inciting secession".
Ahead of Monday's hearing, Chow said she felt uneasy about her "uncertain future" and urged the public to continue to support Hong Kong's pro-democracy protesters, including the 12 young activists detained in Shenzhen.
Wong, meanwhile, said he would not be surprised if they were immediately detained.
"It would be less ridiculous an outcome than others that happened recently in Hong Kong. In the past three weeks, a total of 23 activists, journalists and councillors came under arrest. Every day we have activists standing trial and protesters sent to jail," he said.