A group of Hong Kong activists is demanding a return to British rule as a stepping stone towards independence, as fears grow that Beijing is tightening its grip on the southern Chinese city.

Pro-independence advocates have launched The Alliance to Resume British Sovereignty over Hong Kong and Independence party, the second political group in recent months to advocate a breakaway from China.

"Independence is the ultimate goal, to return to British rule is just a transitional phase," Billy Chiu, the Alliance's leader told AFP Wednesday.

The activists said they believed it would be easier to gain independence from Britain than China.

"An independent nation is Hong Kong's only way out," Chiu said, adding that the new party, which consists of around 30 members, will be formally announced on Sunday.

Chiu in 2013 broke into a People's Liberation Army facility in central Hong Kong holding up a colonial flag and asking the PLA to "get out" of the city.

Hong Kong was handed back to Beijing in 1997, with the Sino-British Joint Declaration preserving its liberties for 50 years.

But there are growing fears its freedoms and semi-autonomous status are under threat as Beijing increases its influence across a range of areas, from politics to the media.

Another group, the Hong Kong National Party, was launched in March, saying it was tapping in to the city's increasing desire to break away from the mainland.

Although not all activists are campaigning for self-rule, some see it as the only solution and have said they are not afraid to use violence to achieve their goal.

The negative sentiment has been exacerbated by incidents such as the recent disappearance of five Hong Kong booksellers, employed by a firm that published gossipy books about leading Chinese politicians.

One of the men, Lam Wing-kee, re-emerged in the city last week and told how he was taken away blindfolded and then kept in a cell, under interrogation and without access to his family or a lawyer, for alleged involvement in bringing banned books into the mainland.

The city's growing pro-independence stance has drawn ire from Beijing and authorities in Hong Kong, who have warned that campaigning for a breakaway will damage the city's future prosperity and could result in unspecified "action according to the law".

But aside from a riot in February that saw running battles with police -- viewed as one of the worst clashes in decades -- pro-independence groups have generally remained quiet.