Members of Hong Kong’s pro-democracy party, Demosisto were detained and questioned by Chinese security agents while travelling between China and the former British colony, the party said on Monday.
The incidents occurred in March and August, the party said in a statement distributed via social media.
It said a news conference would be held at 3 p.m. (0700 GMT) at which further details would be announced.
Demosisto was founded in 2016 by the leaders of Hong Kong’s Umbrella Movement, which came together during the massive pro-democracy protests of 2014, including Joshua Wong.
The party has earned the ire of the local government and Beijing for advocating a popular referendum to determine the political status of Hong Kong, which is now a special administrative region of China after Britain relinquished control in 1997.
Demosisto is also a frequent critic of the ruling Communist Party in Beijing and its allies in Hong Kong.
Chinese security services have long been suspected of monitoring Hong Kong’s pro-democracy groups, although there is little concrete evidence.
However, in 2014, two Chinese security agents were accidentally arrested by local police for following democratic legislator James To, according to a Reuters investigation.