Baton-wielding Hong Kong police fired warning shots and pepper spray early on Tuesday after a riot erupted when officials tried to shift illegal hawkers, the worst clashes since pro-democracy protests in 2014.

Television footage showed one officer in the busy Mongkok district pointing his gun at crowds of protesters who hurled bricks, bottles and pieces of wooden pallets at police during the Chinese New Year clash.

Almost 50 police and 25 protesters were injured.

Police fired at least two warning shots in the air, multiple news outlets reported, a rare occurrence in Hong Kong.

Footage showed protesters digging up bricks from pavements, charging police lines with homemade shields and setting rubbish on fire in the middle of the road.

The street battles erupted after officials tried to move illegal food sellers. Demonstrators, including members of radical "localist" groups, tried to defend the hawkers whom they say add to the festive atmosphere.

Hong Kong leader Leung Chun-ying said the government "strongly condemns such violent acts".

"There was a riot in Mongkok in the early hours of today," the chief executive told reporters. "A few hundred mobs attacked police officers and media."

"They committed acts of arson, threw bricks and other objects at police officers, including those who had already been injured and were lying on the ground."

Footage from Cable Television News showed police and protesters still in a stand-off as day broke while the city's subway operator MTR closed Mongkok station. It later reopened.

Police said they acted after crowds ignored calls to disperse.

"To ensure public safety and public order, police took resolute actions, including using baton and pepper spray, to stop the unlawful violent acts," a statement said, without mentioning any warning shots.

A police spokeswoman said 48 officers were injured, many by glass or by being hit in the head.

She said 23 men and one woman aged between 17-70 were arrested for assaulting police, resisting arrest, disorder in a public place and obstructing police officers.

Journalists injured

Four journalists were also injured, one of which was hit on the head by a brick thrown by rioters, the Hong Kong Journalists Association said in a statement on its Facebook page.

TVB news footage showed protesters harassing and attacking their cameraman who sustained injuries to his hand.

Reports said one of those arrested was Edward Leung, a "localist" candidate for an upcoming by-election on February 28.

The so-called "localists" seek to severely restrict Beijing's influence in the semi-autonomous southern Chinese city. Some even want an independent Hong Kong.

The clashes come at a time when residents are on edge over concerns the city's freedoms are being eroded by China, which resumed sovereignty in 1997.

There is particular concern at the fate of five Hong Kong based-booksellers believed to be detained in mainland China after disappearing last year.

One of them vanished from Hong Kong, fuelling concern he had been abducted by mainland law enforcers who have no right to operate in the city.

Mongkok on the city's Kowloon peninsula was the scene of some of the worst violence during the 79-day "Occupy" pro-democracy street protests in late 2014.

The mass rallies seeking fully free leadership elections in the city blocked some major streets for more than two months.