Her account demonstrates the concerns that many Hongkongers have about the justice system in mainland China spilling over into their city.
Wong told reporters she had been held by Shenzhen authorities without knowing what charges she was facing and that she was interrogated almost every day about the Hong Kong protests.
Following a month and a half in detention, she was sent on a five-day tour of Shaanxi province, where she had to sing the Chinese national anthem and have her picture taken holding the Chinese flag, she said.
Wong was then given police bail on condition she stayed in Shenzhen, she said, but those conditions ended in late September. She returned to Hong Kong earlier this month, and observed a 14-day quarantine before giving Saturday's press conference.
The months-long protests that erupted last year were originally against a proposed extradition bill that would allow for Hongkongers to be sent to mainland China for trial. The bill was shelved, but in June Beijing imposed a harsh national security law on Hong Kong that gives it broad powers to intervene in Hong Kong's legal system, and makes it easier to punish protesters.
Wong also called for the release of 12 activists who were seized by the Chinese coastguard in August allegedly trying to flee to Taiwan on a speedboat.
"I'm worried about the 12 young people; their treatment could be worse than [mine]," she said.