Scores of police officers were hurt Tuesday when a riot erupted in Hong Kong over official attempts to move illegal hawkers, in the most serious violence the city has seen since mass pro-democracy protests in 2014.

Police said nearly 90 of their number were injured, many by broken glass or projectiles, while dozens of protesters were also hurt in the Chinese New Year clash.

Demonstrators levered up bricks from pavements in the busy Mongkok district, charging police lines with homemade shields and setting rubbish on fire in the middle of the road.

One officer was seen pointing his gun at crowds who hurled stones, bottles and pieces of wooden pallet at police.

Officers fired at least two warning shots in the air, multiple news outlets reported, a very rare occurrence in the semi-autonomous southern Chinese city.

They also used pepper spray in chaotic scenes that played out on local television.

As criticism grew of police tactics in the localised disturbance, Commissioner Stephen Lo defended the officer who fired his weapon, saying rioters were continuously attacking his already injured colleague.

"With no alternative, his police colleague used his firearm in accordance with the use of force principles to prevent his fellow colleague from being further attacked," Lo said, adding there would be a full investigation.

Police said 54 protesters aged between 15-70 were arrested for assaulting police, resisting arrest and disorderly conduct in a public place, among other offences.

"We will consider charging the arrested persons for participating in a riot," Lo said. This carries a sentence of up to 10 years in prison.

- 'Fishball revolution' -

The battles, which broke out after officials tried to move illegal food sellers, have been dubbed the "fishball revolution" on social media, after a popular street dish often consumed at Lunar New Year.

Demonstrators, including members of radical "localist" groups -- which stress Hong Kong's separate identity from the mainland -- tried to defend the hawkers, whom they say add to the festive atmosphere.

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

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 (people) attacked police officers and media."

The standoff, which began overnight, lasted into the morning, with the city's subway operator shuttering the local station.

At least four journalists were also injured, one of whom 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.

The so-called "localists" want to restrict Beijing's influence in the city, which was a British colony until its return to China in 1997. Some even want an independent Hong Kong.

The clashes come with tensions high and concerns that the city's freedoms are being eroded by authoritarian China.

There is particular concern at the fate of five Hong Kong based-booksellers who are believed 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.

Leung said there were no plans to cancel Tuesday night's New Year fireworks, when tens of thousands of revellers are expected to gather on either side of the city's Victoria Harbour.

Police said their presence will be increased for the display in the wake of the riot.