Twelve young Hongkongers detained in Shenzhen after being accused of illegally entering mainland waters are at risk of torture, the Hong Kong chapter of human rights group Amnesty International warned on Monday.

"The plight of these 12 young people is extremely concerning. Over the past 100 days, they have been subjected to some of the most common tactics from the Chinese police’s playbook," the group said.

"They have been deprived of their basic human right to fair trial, including the chance to select their own lawyers, and they are at risk of torture and ill-treatment. Their families have repeatedly been denied access to them, and at least six lawyers have been threatened by the Chinese authorities to make them drop this case."

Amnesty called on Beijing to allow the detainees access to their families and chosen lawyers and to ensure that they are not ill-treated.

The 12 were intercepted on August 23 as they allegedly tried to flee from Hong Kong to Taiwan by speedboat.

Last week, police in Shenzhen said they had completed their investigation into the alleged illegal border crossing and had handed the Hongkongers over to prosecutors.

Mainland lawyers told RTHK that a trial could be held for the 12 from as early as next month, although it could take half a year before they are brought to court.