Joshua Wong Chi-fung, co-founder of student activist group Scholarism, posted on his Facebook page on Sunday that he was followed when travelling in Taiwan.
Joshua went to Taiwan with two other Scholarism members on Friday. According to the 17-year-old, four people followed them and took photos.
The three returned to Hong Kong earlier than planned because they said they feared for their safety.
At the airport, Joshua and his friends noticed a man filming them with a digital video camera. After confronting the man, he claimed he had been hired to film Joshua to find out where he was going.
Another man following the group, also confronted by Joshua, said a client from Hong Kong had paid him to capture images of Joshua if he visited a nightclub. The client had provided the man with Joshua's hotel address.
"I'm always above board. Just tell me if you want to take photos of me," wrote Joshua.
He said he suspects the move may be related to the upcoming Occupy Central protests. Joshua has been vocal in his support for electoral reform. He called for all activists to be careful, as now they are under scrutiny.
"They do it on purpose to [cause you] psychological stress," said Joshua. "What worries me is that once they have your schedule, what's their next move?"