A new brand of political activist is taking centre stage in Hong Kong, pushing for violent protest and even independence for the city, where the case of disappeared booksellers has fuelled mistrust of China.

A seemingly innocuous rally to protect illegal hawkers from health inspectors earlier this month descended into running battles with police in the worst clashes for decades.

Masked protesters hurled bricks, police fired warning shots and the streets were left ablaze in the commercial district of Mong Kok.

Leading the protest were young "localists", a term coined for radical groups that grew out of the failure of massive pro-democracy rallies in 2014 to win concessions from Beijing on political reform.

They say the recent violence was borne out of frustration with authorities in Hong Kong and Beijing who refuse to listen to their views.

"Our way of protest is a forceful way and an effective way to put pressure on the Beijing government," Edward Leung of Hong Kong Indigenous, created in 2015, told AFP.

"A war or a battle is inevitable."

The Mong Kok violence also came at a time of growing fears among the population at large that the semi-autonomous city's freedoms are ebbing away -- an impression that has deepened with the disappearance of five Hong Kong booksellers known for publishing titles critical of Beijing.

Four of the men are now under criminal investigation on the mainland and the fifth says he is in China "assisting" authorities.

- 'Hong Kong nation' -

Protesters in Mong Kok drew the ire of Beijing, which branded them "separatists".

Leung, a 24-year-old philosophy student, is unafraid of the label.

"Our ultimate goal is to build a Hong Kong nation," he says, describing the group's followers as predominantly 20-somethings, a mix of students, freelancers and the unemployed.

The localist movement is nebulous, mainly comprising small groups galvanising support online, with activists keen to keep their identities secret.

But more established radical pro-democracy groups are also now aligning under the "localism" banner.

Veteran social activist Wong Yeung-tat, founder of Civic Passion, set up in 2012, says he does not want to see violence, but describes the riots as a watershed.

"Many protesters think it's time to fight back," says Wong, 37.

"In the end, maybe we have to face fighting the revolution."

Leung was arrested during the Mong Kok street battles, which left more than 100 people injured, and faces a rioting charge alongside more than 30 other participants, with a maximum 10-year prison sentence.

He and other localist groups are also trying legal means to get their message across -- Leung will stand in a by-election for a parliamentary seat at the end of February, though is unlikely to win.

"Localism has growing influence among young people but not to the extent that they can do something to upset the system," says Chung Kim-wah, a professor of social sciences at the Polytechnic University of Hong Kong.

But if authorities remain intransigent they will fuel the fire, Chung adds, particularly if they use force against protesters.

The Hong Kong government faces a delicate balancing act between keeping the peace and satisfying Beijing.

"From Beijing's perspective, even a minority of radical localists is unacceptable," said Sonny Lo, professor of social sciences at the Hong Kong Institute of Education.

"Beijing is eager to see a more hardline position adopted by the Hong Kong government."

- Chinese 'occupation' -

Since Hong Kong was handed back to China by Britain in 1997, with an agreement its freedoms be protected for 50 years, the city has been feisty, with regular protest rallies that attract tens of thousands of people and have remained largely peaceful.

But with distrust of Beijing growing, tensions are spiralling, worrying those who have valued the city's stability.

"Hong Kongers have always wanted to live peacefully. Even if the government is not doing good, one should not do this kind of thing," said one 76-year-old retiree surnamed Chan referring to the recent riots.

"You can use violence to fight for Hong Kong," added salesman Chan Yat-ho, 24.

"But can violence really help you get what you want?"

Some localists, however, see no other way.

"The Chinese Communist Party is a terror group," one activist told AFP on condition of anonymity.

"Whatever we do to resist their occupation of Hong Kong shouldn't be labelled as violent."