Police and protesters have clashed at Hong Kong's international airport after flights were suspended for a second day due to blockades.

Online footage showed police armed with batons and pepper spray tackling demonstrators who have taken over the airport's terminals as part of their push for democratic reforms.

After a brief respite early Tuesday during which flights were able to take off and land, the airport authority announced check-in services for departing flights were suspended as of 4.30pm.

The authority advised people not to come to the airport, one of the world’s busiest transport hubs.

On Monday, more than 200 flights were cancelled and the airport was effectively shut down with no flights taking off or landing.

Passengers have been forced to seek accommodation in the city while airlines struggle to find other ways to get them to their destinations.

The airport disruptions are an escalation of a summer of demonstrations aimed at what many Hong Kong residents see as an increasing erosion of the freedoms they were promised in 1997 when Communist Party-ruled mainland China took over what had been a British colony.

Police have arrested more than 700 protesters since early June and say they have infiltrated the ranks of the demonstrators, leading to concerns that officers were inciting violence.

Scores of people have been injured, both protesters and police, including a woman reported to have had an eye ruptured by a beanbag round fired by police during clashes on Sunday.

Police said they are investigating the incident, which protesters have taken up as a rallying cry.

Some of those joining in the airport occupation wore gauze bandages dyed with artificial blood over one eye.

The United Nations’ top human rights official condemned violence surrounding the protests and called on the authorities and protesters to settle their dispute peacefully.

Hong Kong ProtestsA Dragonair plane flies past a road sign at the airport in Hong Kong (Kin Cheung/AP)

Rupert Colville, spokesman for UN High Commissioner for Human Rights Michelle Bachelet, said her office had reviewed evidence that police are using “less-lethal weapons in ways that are prohibited by international norms and standards”.

That includes firing tear gas canisters into crowded, enclosed areas and directly at individual protesters, “creating a considerable risk of death or serious injury”, Mr Colville said in a statement.

The commissioner urged both sides to engage in “open and inclusive dialogue”, which is the “only sure way to achieve long-term political stability”. it said.

Hong Kong ProtestsTravellers react as they try to break through the protesters at the departure gate (Kin Cheung/AP)

Hong Kong leader Carrie Lam said the instability, chaos and violence have placed the city on a “path of no return”.

The demonstrators have shown no sign of letting up on their campaign to force Ms Lam’s administration to respond to their demands, including that she step down and entirely scrap proposed legislation under which some suspects could be sent to mainland China, where critics say they could face torture and unfair or politically charged trials.

Hong Kong ProtestsA protester shows a placard to stranded travellers (Kin Cheung/AP)

Ms Lam has rejected all calls for dialogue, part of what analysts say is a strategy to wear down the opposition through police action while prompting it to take more violent and extreme actions that will turn the Hong Kong public against the protest movement.

At the airport, protesters discussed among themselves whether they should prevent all would-be travellers from entering, with some saying their presence there was meaningless unless they blocked all access to the facilities.

Demonstrators have in recent days focused on their demand for an independent inquiry into what they call the police’s abuse of power and negligence.

That followed reports and circulating video footage of violent arrests and injuries sustained by protesters.

Some protesters have thrown bricks, eggs and flaming objects at police stations.

Police say several officers have suffered burns, bruises and eye damage inflicted by protesters.

Ms Lam told reporters that dialogue would only begin when the violence stopped.

She reiterated her support for the police and said they have had to make on-the-spot decisions under difficult circumstances, using “the lowest level of force”.

“After the violence has been stopped, and the chaotic situation that we are seeing could subside,” Ms Lam said.

“I as the chief executive will be responsible to rebuild Hong Kong’s economy … to help Hong Kong to move on.”

She did not elaborate on what steps her government will take toward reconciliation.

After two months, the protests have become increasingly divisive and prompted clashes across the city.

The airport shutdown added to what authorities say is already a major blow to the financial hub’s crucial tourism industry.

Hong Kong Protests Photo GalleryProtesters cover their eyes during a demonstration at Hong Kong International Airport (Kin Cheung/AP)

Kerry Dickinson, a traveller from South Africa, said she had trouble getting her luggage Tuesday morning.

“I don’t think I will ever fly to Hong Kong again,” she said.

The protests early on were staged in specific neighbourhoods near government offices.

However, the airport protest has had a direct impact on business travel and tourism.

Analysts said it could make foreign investors think twice about setting up shop in Hong Kong, which has long prided itself as being Asia’s leading business city with convenient air links across the region.

The black-clad protesters Tuesday held up signs in Chinese and English to appeal to travellers from mainland China and other parts of the world.

“Democracy is a good thing,” said one sign in simplified Chinese characters, which are used in mainland China instead of the traditional Chinese script of Hong Kong.

Adding to the protesters’ anger, Hong Kong’s Cathay Pacific Airways told employees in a memo that the carrier has a “zero tolerance” for employees joining “illegal protests” and warned violators could be fired.

Hong Kong ProtestProtesters gather near an information board (Kin Cheung/AP)

While China has yet to threaten sending in the army, as it did against pro-democracy protesters in Beijing in 1989, the exercises in Shenzhen were a demonstration of its ability to crush the demonstrations, even at the cost to Hong Kong’s reputation as a safe haven for business and international exchange.

Images on the internet showed armoured personnel carriers belonging to the People’s Armed Police driving in a convoy towards the site of the exercises just across the border from Hong Kong.

The People’s Liberation Army also stations a garrison in Hong Kong, which recently released a video showing its units combating actors dressed as protesters.

The Hong Kong police on Monday also put on a display of armoured car-mounted water cannons that it plans to deploy by the middle of the month.