A police crackdown on illegal street vendors in Hong Kong worsened into overnight rioting in one of the busiest districts in Asia's financial capital.

Riot police clashed with protesters and fired two warning shots as protesters launched missiles and set fires in the working-class district of Mong Kok.

"Because many rioters were attacking police with hard objects and seriously threatened their lives, there was no choice but to protect colleagues," Crusade Yau Siu-kei, deputy Mong Kok district commander, told the South China Morning Post.

Rubbish bins, chunks of brick and broken bottles were scattered along the Nathan Road shopping strip on Tuesday morning, Reuters reported.

The street violence reminiscent of the pro-democracy protests in late 2014 was triggered when police cleared hawkers who peddled local delicacies from street-side stalls.

More than a hundred protesters broke loose in Portland Street at 2 am, forcing police to engage them.

With the area descending into chaos, the underground train station in Mong Kok was closed.

While 23 people, including a woman, were arrested for assaulting a police officer, three police officers were treated in a hospital for minor injuries.

At least four journalists filming chaotic scenes were also injured in clashes. Police have cautioned motorists to avoid restive areas.

The street protests, which received instant social media backing, also spread to areas surrounding Shantung and Soy Streets, local reports said.

Several parts of the city were in lockdown on Tuesday morning, as protesters, including the so-called 'localists' who hit the streets in 2014 demanding independence for Hong Kong, the SCMP reported.