A Hong Kong pro-democracy group led by high-profile activist Joshua Wong said Monday two of its members had been detained and questioned in mainland China.
The pair were released within hours but were warned not to publicise their experiences, the Demosisto party said in a statement.
Hong Kong has rights unseen on the mainland but concerns are growing that those liberties are under serious threat as China tightens its grip on the semi-autonomous city.
Wong described the detentions as a "clear signal" that Chinese security forces were trying to suppress the democracy movement.
Demosisto campaigns for self-determination for Hong Kong, a concept which Beijing considers intolerable and a challenge to its territorial integrity.
The pair, who according to the statement were not key members of the party, were allegedly picked up at train stations in the southern Chinese towns of Shenzhen and Guangzhou in March and August.
Both members were asked about their participation in recent protests and for information about Demosisto, the group said.
The first was questioned at a police station and released around three hours later, according to the statement.
The second was taken to a hotel, where they were attached to what the group said resembled a lie detector and accused of "trying to stir up trouble in the mainland".
Demosisto said the member was offered payment to provide information after returning to Hong Kong, before being asked to sign an apology and released after five hours.
Beijing considers calls for self-determination in Hong Kong as tantamount to demands for independence.
The emergence of an independence movement in the last four years has led to a crackdown in the city, with some activists jailed and others barred from office.
It also comes as Hong Kong prepares to launch a high-speed rail link to the mainland at the end of September.
The new line will see China's security staff operating on the city's turf for the first time, prompting fresh fears over citizens' freedoms.
Demosisto founder Wong said the rail link could enable Chinese state security to "fully review and follow... every citizen" who visits the mainland.