The 11 men and one woman were captured by the Chinese coastguard on Aug. 23 aboard a speedboat believed to be bound for Taiwan.

All had faced charges linked to the protest movement embroiling Hong Kong, including rioting and violation of the a national security law China imposed in June.

Family members and supporters of some of the 12 hiked to the peak of Kat O island in Hong Kong's remote northeastern reaches, looking onto China's high-tech boomtown of Shenzhen, and the Yantian district where the dozen are being held.

Some peered through binoculars at a hill where the detention centre is located. Several told Reuters they want the Chinese authorities to deal with the cases in a just, fair and transparent manner.

The group inflated blue and white balloons and wrote the names of the detainees on them, before releasing them into a leaden sky. They chanted for their "immediate safe return" while holding white banners reading "SAVE 12" and "Return Home".

"I hope he can see the balloons and know we didn’t give up yet," said the 28-year-old wife of detainee Wong Wai-yin.

A Hong Kong marine police vessel later docked on the island, with police questioning and taking down the details of several reporters present.

Authorities have denied family and lawyers access to the 12, insisting they be represented by officially appointed lawyers. Last week seven detainees wrote handwritten letters to their family, but the group said in a statement that "they seem to have been compiled under duress".

Eddie Chu, a former lawmaker who recently quit his post in protest against political suppression by authorities under the national security law, said it was important to keep fighting.

"We are so close to them, just a few kilometres in reality, but in fact it's like ... something unreachable. So we need to have the balloons to do this for us.”