Lawyers representing some of the Hong Kongers captured by the Chinese coastguard as they allegedly tried to flee the financial hub by speedboat told AFP Monday that they have been denied access to their clients.
Mainland authorities announced a group of 12 Hong Kongers were intercepted some 70 kilometres (43 miles) southeast of the city on August 23 trying to escape by boat, and were handed over to police in neighbouring Shenzhen.
They have since disappeared into authoritarian China's opaque judicial system.
Neither mainland nor Hong Kong authorities have publicly confirmed who has been arrested.
But Hong Kong media outlets -- including a pro-Beijing newspaper -- have identified some of those on board, while relatives of those detained have also come forward.
Some of those identified as being on the speedboat were facing prosecution for involvement in pro-democracy protests last year.
One man -- Andy Li -- was recently arrested under a new security law Beijing imposed on Hong Kong.
Another is a dual national with Hong Kong and Portuguese citizenship.
On Monday, two mainland-based lawyers told AFP they had tried and failed to visit their clients after being instructed by family members.
"They said I can't prove that the instructions I have came from family members even though I have provided my client's birth certificate issued in Hong Kong," said Ren Quanniu, a lawyer who travelled nearly 1,500 km (over 900 miles) from central China to Shenzhen.
Ren said he also visited the police officer in charge of the case, who refused to receive legal documents -- including a written request for his client Wong Wai-yin to be handed back to Hong Kong jurisdiction.
He said he then filed a complaint with prosecutors in Shenzhen.
Lu Siwei, another lawyer, said he had a similar experience when he tried to visit his client in detention last week.
Ren and Lu both said Shenzhen police were treating the case as an "illegal border crossing", an offence that carries up to a year in jail.
However, Lu said police informed him some of the detained may also face the more serious charges of "organising others to cross the border illegally", which carries sentences up to life in jail.
Ren said other lawyers he knew were representing the arrestees were also being denied access.
The prospect of Hong Kongers getting entangled in authoritarian China's judicial system was the spark that lit seven months of huge and often violent pro-democracy protests last year.
The movement began in response to plan to allow extraditions to the mainland and soon morphed into calls for democracy and greater police accountability.
Since then authorities have cracked down on the city's democracy movement, prosecuting thousands of protesters as well as leading activists.
In June, Beijing blanketed semi-autonomous Hong Kong in a new security law which toppled the legal firewall between the two.
Under the law, China has claimed jurisdiction for serious national security crimes and empowered its security agents to operate openly in Hong Kong for the first time.