Hundreds of Hong Kong pro-democracy activists blocked some routes to the city's airport Sunday as the financial hub began cleaning up after another night of violent protests marked by fires, tear gas and police beatings. Operators of the Airport Express train said it suspended services on Sunday afternoon, while black-clad protesters - hiding from CCTV cameras under umbrellas - built barricades at the airport bus station and attempted to stop traffic on the main road leading to terminals.Outside one terminal at the international hub, protesters set off fire extinguishers, piled luggage trolleys into makeshift road barricades and smashed surveillance cameras.Hong Kong police said they were poised to launch a "dispersal operation", and in a Facebook post warned protesters to leave the airport area "immediately". Flights had so far not been disrupted, but some passengers were left bewildered by the chaotic scenes. "I want to go into the airport to find my sister, but I couldn't get in," Indonesian domestic helper Samirah told AFP.The airport is covered by an injunction banning protesters from entering, imposed after a shutdown in August which ended in ugly clashes.But protesters have routinely ignored legal moves to ban their actions since the anti-government movement sprang to life three months ago.The protests were sparked by an attempt by Hong Kong's Beijing-backed government to pass a now-shelved extradition bill, but have expanded into a wider pro-democracy push.On Sunday, city sanitation workers were seen clearing debris and removing graffiti after a night of pitched battles between protesters and police.Hours earlier, a huge fire burned in the city's commercial district as chaos ripped through the centre of a city usually renowned for its stability and prosperity."I'm really, really tired. I think many Hong Kong people had a sleepless night yesterday," said 18-year-old student protester called May."I almost couldn't manage to get up, but I'm determined to go today."Hardcore demonstrators Saturday hurled petrol bombs at government buildings and police, who responded with tear gas and water cannon laced with chemical dye before making mass arrests inside the city's underground metro stations. Warning shotsVideo footage captured by local media showed elite police charge a crowd cowering inside a train carriage -- with one man, drenched in pepper spray, crying in anguish on his knees as he tried to protect his female friend.Officers left the carriage without making any immediate arrests. Anger at police tactics swirled across social media."The police are a licenced mob, they have the licence to attack and to assault," Kwok Ka-ki, a pro-democracy lawmaker, told AFP. "The government is no different from an autocratic regime."Decrying the "horrifying" police raid on the train carriage, Amnesty International called for a probe into police conduct."Violence directed at police... is no excuse for officers to go on the rampage elsewhere," Man-kei Tam, director of Amnesty International, Hong Kong said in a statement. Police said officers also fired two warning shots into the sky after being attacked by a group of "violent protesters who attempted to once snatch police pistols"."With such escalating violence and progressively lethal weapons of protesters, the safety of police officers and other members of the public is seriously threatened," police said in a statement. The hospital authority said Sunday that 31 people were admitted with injuries following the clashes, including five who remain in a serious condition. High-profile arrestsChina's state news agency Xinhua posted a video on Twitter late Saturday of armed Chinese riot police holding "anti-riot drills in Shenzhen", a city which borders Hong Kong.Beijing has also unleashed a mix of intimidation, propaganda and economic muscle in an attempt to muzzle the movement, which it views as a direct challenge to its rule.Police on Friday rounded up several high profile pro-democracy activists and politicians in sweeping arrests that rights groups said was taken directly from Beijing's playbook.Police denied the sweep was timed specifically to weaken the weekend's protests.More than 900 people have been arrested since June in connection with the protests.Saturday marked the fifth anniversary of Beijing's rejection of universal suffrage for Hong Kong that sparked the 79-day "Umbrella Movement" in 2014.That petered out without the government making any major concessions.