Streets in downtown Hong Kong were littered with debris and walls were covered in graffiti on Wednesday as the city woke up from violent clashes on Christmas Eve. Undercover officers fought with protesters, some wearing Santa hats, in the streets of the city’s shopping districts, the working-class Mong Kok area and outside the Peninsula Hotel in Tsim Sha Tsui. The evening's unrest was the most sustained in what has otherwise been a few weeks of comparative calm for a city upended by more than six months of violent protests. Thousands of black-clad protesters – some wearing Santa hats and reindeer antlers – took to the streets of Tsim Sha Tsui, a usually bustling tourist district. Clashes soon erupted, with riot police firing multiple rounds of tear gas to disperse protesters throughout the evening, including outside the famous Peninsula Hotel. Rubber bullet casings were also seen on the ground. Police said a "large group of rioters" had built barricades, damaged traffic lights and dug up bricks on the area's major thoroughfares and vandalised stores. Protesters smashed shop windows, while police responded with tear gas and the arrest of a number of demonstrators. At least 25 people were injured on Christmas Eve, including one in a serious condition, hospital authorities said. For the past six month, mass anti-government protests have rocked the semi-autonomous Chinese city. Hong Kong’s pre-holiday strife extended the months of clashes between protesters and police, ignited by a proposed extradition law to allow fugitives to be sent to China to stand trial. The unrest has also pushed the city into its first recession in a decade. Though the bill was withdrawn, protests have persisted with more demands, including direct elections of the city’s leader. The protests demanding greater democratic rights show no sign of ending despite the overwhelming victory by anti-establishment candidates in elections for district representatives earlier this month. Protests will likely continue into the new year. Civil Human Rights Front, the organizer of some of Hong Kong’s biggest peaceful protests, is calling for a march through the city’s centre on January 1. The organiser said it has applied for a police permit. Meanwhile, flash mob rallies were held in multiple malls across the financial hub, with protesters chanting anti-government slogans. In Harbour City, a luxury mall, police used pepper spray and batons when a group of plainclothes officers were discovered and surrounded, an AFP reporter at the scene said. The plainclothes officers made multiple arrests as the crowds threw objects and heckled them. Riot police quickly arrived, with one aiming a shotgun at protesters as shops quickly shuttered. Hong Kong's many malls have become regular protest venues as protesters try to cause economic disruption in their push for greater democratic freedoms and police accountability. Hong Kong’s Chief Executive Carrie Lam wished Hong Kongers a “safe, happy and peaceful Christmas” on Wednesday. “Christmas is a time for everyone to celebrate. I wish everyone in HK to have a safe, happy and peaceful Christmas,” she said. Protesters have been calling on Ms Lam to quit over her push to pass the extradition bill. A former British colony with a sizeable Christian population, Hong Kong is having a distinctly muted Christmas this year. Christmas Eve is usually a major night for retailers and bars, with key districts pedestrianised. But police said they would not close roads to traffic this year, fearing protesters might use the opportunity to gather.