Hong Kong riot police fired warning shots during angry clashes, that emerged after authorities tried to remove illegal street stalls set up for Lunar New Year celebrations, on Tuesday.
Protesters prised bricks from the sidewalk to hurl at police, while others toppled street signs and set fire to rubbish bins.
The police spokeswoman said 24 protesters were arrested, and as many as 48 police were injured in the clashes. The underground train station was also closed temporarily.
The clashes broke out after police moved in to clear "hawkers", or illegal vendors who sell local delicacies, trinkets and household goods from makeshift streetside stalls.
The hawkers, quickly mobilised on Twitter under the hashtag #FishballRevolution.
Mong Kok, across the harbour from the Asian financial centre, is a bustling shopping district packed with street markets, shops and high-rise residential buildings,
Police said two warning shots were fired into the air, with pepper spray and batons to disperse the crowd.
The shots were fired as protesters surrounded several traffic police, pelting them with bricks and bottles.
Protesters were reported shouting, "Establish a Hong Kong country!" during running battles with the police.
Hong Kong police declined to comment on who had been involved in the protests or to confirm who had been arrested.
Secretary for Security Lai Tung-kwok said police were investigating "indications" that the clashes had been organised.
Hong Kong Chief Executive Leung Chun-ying told reporters at a hastily called news conference that the city's government strongly condemned the violence.
Hong Kong Indigenous, a "localist" group that is fielding a candidate in a Legislative Council by-election in a few weeks, was involved in the protest, the South China Morning Post newspaper reported.
The group said that its candidate, Edward Leung Tin-kei, had been arrested. They could not be reached immediately by telephone to confirm his arrest.
Radical protesters and "localists" demanding greater Hong Kong autonomy have vowed to keep fighting even as China shows signs of tightening its grip.
The protests have been the worst street violence since pro democracy protests in 2014. Beijing's greatest political challenges in decades, pro-democracy protests were organised to demand greater democracy in the former British colony that returned to Beijing rule in 1997.