More than 30 protesters appeared in court in Hong Kong Thursday charged with rioting after the worst clashes the city has seen since mass pro-democracy protests.

The violence erupted after officials tried to clear illegal street hawkers from the busy commercial neighbourhood of Mong Kok late Monday night.

Protesters gathered to defend the stallholders and police tried to disperse the crowds -- the ensuing confrontations left over 100 people hospitalised and shocked the city.

Police fired warning shots in the air during the clashes -- a very rare occurrence in Hong Kong -- while demonstrators levered up bricks from pavements, charged police lines with homemade shields and set rubbish on fire.

Each of the 37 protesters at magistrate's court Thursday faced a single charge of participating in a riot, with a sentence of up to 10 years.

Many of the defendants shielded their identities with masks and hoods outside court and friends and relatives packed the hearing.

Some had visible head injuries and their lawyers said they wanted to lodge complaints against police.

"A plain-clothes officer hit him in the chest three times at the police station," one lawyer said of his client Chan Ho-man, 17.

All were granted bail but were banned from parts of Mong Kok. They will next appear on April 7.

A 15-year-old boy faces a rioting charge in juvenile court Friday.

Student leader Joshua Wong, a key organiser of the 2014 pro-democracy rallies, observed the hearing -- he did not participate in Monday's protests.

"The pro-establishment side needs to reflect on why some of the youth were ready to be put in jail for a maximum 10 years by joining the riots," he told AFP.

"If more activists are arrested... it will just motivate more of the new generation to continue to have more radical action," Wong added, saying some of the arrests were unfair as a number of the participants were non-violent.

- 'Fishball revolution' -

Wong said fellow activist Derek Lam, from his Scholarism student protest group, was arrested at Hong Kong airport Wednesday on his way to Taiwan with his family over the riot, and was being held by police.

Wong said Lam had been a peaceful participant in the demonstration. Police said they would not comment on individual cases.

At least four journalists were injured in the violence. One was filmed being beaten by officers and has reported the incident to police.

The battles have been dubbed the "fishball revolution" after a favourite Hong Kong street snack and reflect underlying tensions over the erosion of the city's traditions.

Demonstrators included "localist" activists who want to restrict Beijing's influence on the city.

Semi-autonomous Hong Kong was returned by Britain to China in 1997 with its way of life protected for 50 years by a joint agreement.

But there are fears that freedoms enshrined in the agreement are being eroded by Chinese influence, including the recent case of five Hong Kong publishers known for titles critical of Beijing, four of whom it is confirmed have been detained on the mainland.