Hong Kong police on Saturday arrested a group accused of helping 12 pro-democracy activists detained in mainland China for nearly two months after attempting to flee to Taiwan in a speedboat.

The group of nine, some of whom had been previously arrested for participating in the massive protests that threw the financial hub into turmoil last year, were taken into custody for "assisting offenders".

"Their roles include ownership of the boat, financial backing, providing accommodation before the trip, transportation to the pier and arranging their lives after arriving in Taiwan," a police spokesperson told reporters.

China's coast guard intercepted a boat transporting the 12 Hong Kongers to Taiwan in late August. The self-ruled island routinely offers sanctuary to people attempting to flee the authoritarian mainland China.

Some of those on board were facing prosecution in Hong Kong for activities linked to last year's huge violent pro-democracy protests.

Beijing imposed a new security law on in late June that stepped up penalties against dissent and allowed mainland security agents to operate openly in the city.

The incident became the latest to compound fears among many Hong Kongers of authoritarian China's growing reach into their lives.

Opposition leaders have accused Hong Kong's pro-Beijing government of conspiring to subject the 12 activists to mainland China's opaque and Communist Party-controlled justice system.

Police in Hong Kong have said they played no role in the arrest of the group.

AFP