Hong Kong riots in Mong Kok district
Bricks dug out from pavements are seen left on a chair after protesters clashed with riot police at Mongkok district in Hong KongReuters

Hong Kong authorities have arrested a key Occupy movement activist for his suspected involvement in the latest unrest in Mong Kok district as calm returns to the city. The Hong Kong police have said that an inquiry will be done on the warning shots fired by officers.

There were intense protests at Mong Kok after police tried to remove illegal food stalls from a major junction, eventually leading to riots. Officials have not ruled out whether the unrest was masterminded by pro-democracy groups in the district.

As part of the latest crackdown, law enforcement agencies have nabbed Lam Shun-hin, a prominent member of the student-dominated protest group Scholarism, at Hong Kong International Airport. The group confirmed his arrest and demanded his immediate release. A spokesperson for Scholarism told the South China Morning Post: "He left Mong Kok at about 2.15am on Tuesday morning, and did not attack any policemen or did anything violent."

Students have denied taking part in the latest protests, which later gradually turned into riots. Despite the virtual lockdown of the district for nearly 26 hours, no less than 90 people, including some police officers were injured. Dozens remain in custody on suspicion that they could have played a pivotal part in inciting violence. The clashes were the worst since the days of the pro-democracy Umbrella Movement's rallies in the district.

"Rioters attacked a police officer with hard objects and threatened his life. He fell on the ground but kept being attacked by the rioters. 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 and also for his own personal safety," Police Commissioner Stephen Lo told reporters at a press conference adding that an investigation was underway.

Meanwhile, food stalls and restaurants started reopening in the busy streets of Mong Kok owing to the three-day Chinese New Year holiday. "It's their business to protest, my business is another matter... I stayed [on Monday night] till the clashes started. I have been doing this every year – I can't remember how many years... more than 20 years," a food stall owner named Chan told Hong Kong Free Press.