Flights have been suspended at Hong Kong's international airport, the airport authority said on Tuesday, citing disruptions caused by anti-government protests.

Tuesday's suspension comes a day after a pro-democracy protest brought the air transport hub to a standstill.

"Terminal operations at Hong Kong International Airport have been seriously disrupted as a result of the public assembly," the airport authority said in a statement on Tuesday.

"Members of the public are advised not to come to the airport."

The city's leader Carrie Lam denounced the demonstrations, saying that "lawbreaking activities in the name of freedom" were damaging the rule of law, and that the Asian financial hub's recovery from anti-government protests could take a long time.

Later on Tuesday, protesters were seen gathering once more in the departure hall of the airport, with Associated Press reporting some of the departing travellers struggling to get past the sitting demonstrators and into the immigration section.

Al Jazeera's Rob McBride, reporting from Hong Kong airport, said the airport staff have been trying to clear the backlog.

"There have been calls from the protesters to once again build up numbers and do what they did on Monday. We haven't seen the numbers that we saw yesterday," he said.

"We are seeing as the afternoon goes on more and more protesters arriving," he added.

On Monday, airport authorities cancelled all outgoing flights, as thousands of protesters started to gather in the departure and arrival areas of the airport.

However, the South China Morning Post reported that as many as 160 outbound and 150 inbound flights were already cancelled for the rest of the day on Tuesday before the official announcement came later in the day.

'Violent crackdown'

The airport protest was a rare case of the movement having a direct impact on business travel and tourism - mainstays of Hong Kong's economy.

Many protesters said on Monday that they were angered by Sunday's police crackdown, which injured several people and at least one female medic was blinded in one eye.

The protests, which have seen both sides adopt increasingly extreme tactics, had until Monday been mostly confined to neighbourhoods across the former British colony.

Police officers were seen firing tear gas at protesters inside a train station, while beating some with their batons.

There have also been allegations of authorities impersonating protesters and trying to stir up violence to draw the attention of the riot police.

As this developed, China has ratcheted up its rhetoric, accusing demonstrators of "wantonly" trampling on Hong Kong's rule of law and social order.

UN raises concerns

The United Nations said in an official statement on Tuesday is it worried about the escalation in Hong Kong, adding that the organisation calls for the two sides to engage in dialogue.

"The High Commissioner condemns any form of violence or destruction of property and urges everyone participating in the demonstrations to express their views in a peaceful way. She notes the Chief Executive’s commitment to 'engage as widely as possible' and to 'listen to the grievances of the people of Hong Kong.'," the statement by Michelle Bachelet said.

"She calls on the authorities and the people of Hong Kong to engage in an open and inclusive dialogue aimed at resolving all issues peacefully. This is the only sure way to achieve long-term political stability and public security by creating channels for people to participate in public affairs and decisions affecting their lives" it continued.

The UN also said it was imperative authoroties investigate evidence of police forces firing tear gas at protesters in ways banned by international law.

"The UN Human Rights Office has reviewed credible evidence of law enforcement officials employing less-lethal weapons in ways that are prohibited by international norms and standards," the statement read.

"Officials can be seen firing tear gas canisters into crowded, enclosed areas and directly at individual protesters on multiple occasions, creating a considerable risk of death or serious injury," it added.

The statement concluded saying the authorities should "ensure that the right of those who are expressing their views peacefully are respected and protected, while ensuring that the response by law enforcement officials to any violence that may take place is proportionate and in conformity with international standards on the use of force, including the principles of necessity and proportionality".

'Dangerous situation'

Following Monday's sudden airport closure, Lam said: "Violence, no matter if it's using violence or condoning violence, will push Hong Kong down a path of no return and push society into a very dangerous and worrying situation."

"The situation in HK in the past week has made me worried we have reached this dangerous situation," she added.

Her comments were echoed by Yang Guang, spokesman for the Hong Kong and Macao Affairs Office of the State Council.

"Hong Kong's radical demonstrators have repeatedly used extremely dangerous tools to attack police officers, which already constitutes a serious violent crime, and also shows the first signs of terrorism emerging,", Yang said at a press briefing in Beijing on Monday.

Some Hong Kong legal experts say official descriptions of some protesters' actions as "terrorism" could lead to the use of extensive anti-terror laws and powers against them.

Yang did not suggest that the Chinese government has any imminent plans to step in, but later on Monday, two state media outlets - the nationalistic tabloid Global Times and the People's Daily - published a video of armoured personnel carriers purportedly driving towards Shenzhen, a city bordering the semi-autonomous territory.

The People's Armed Police (PAP) are in charge of "handling riots, turmoil, seriously violent, criminal activities, terrorist attacks and other societal security incidents", the People's Daily said in the text accompanying the video.

Hong Kong has witnessed 10 weeks of mass anti-government protests that were sparked by a now-shelved extradition bill which would allow criminal suspects to be extradited to mainland China.

The demonstrations have since expanded into wider demands for democratic reforms, a call for Chief Executive Carrie Lam to resign and an independent inquiry into police conduct during the demonstrations.