The 12 were arrested on Aug 23 for illegal entry into mainland Chinese waters after setting off from Hong Kong in a boat bound for self-ruled Taiwan.

All were suspected of committing crimes in Hong Kong related to the anti-government protests that erupted last year. Ten had been charged, released on bail and not allowed to leave the former British colony, and all are now being detained in neighbouring Shenzhen.

Relatives of some of the detainees held a news conference outside the Hong Kong police headquarters on Sunday to express their frustration with local authorities.

"We want our son back...even though we can't visit him, at least give us a letter from him to confirm that he's there," the father of one detainee, Li Tsz Yin, said.

The Hong Kong government and the police did not immediately respond to a request for comment from Reuters.

The relatives also asked police "to give an account of the date, time, place and process of the arrest" and whether there were any injuries or casualties, and the Marine Department to release radar records of the day of the arrest.

They said the government "only shirked responsibility and confused the public with mere excuses."

"However, up to now, the lawyers appointed by the families have been refused to meet with the detainees. In other words, the conditions of the so-called arrested persons are still known only to the Chinese authorities," a statement said.

On Tuesday, Hong Kong leader Carrie Lam voiced discontent with the group being characterised by some as "democratic activists being oppressed", saying they were running away from the law. Lam said they would have to be "dealt with" by mainland authorities, but pledged to provide "feasible" assistance.

Police in Shenzhen said last Sunday they were suspected of illegal entry, their first public comment on the matter. The same day, China's foreign ministry labelled the group as "separatists".