Flight check-in services have been suspended at Hong Kong's international airport, the airport authority said on Tuesday, citing disruptions caused by anti-government protests. All departing flights are cancelled for the remainder of the day.

The authority said it did not expect arriving flights to be affected, though dozens of arriving flights were already cancelled.

Protesters clogged the departure area after the airport was temporarily reopened Tuesday morning, forcing a second shutdown.

After filling up two separate arrivals halls, demonstrators have streamed into the departure area despite increased security measures designed to keep them out. Passengers struggled to get past the sitting protesters and into the immigration section.

Some flights were able to take off earlier Tuesday, relieving some of the pressure from the cancellations of more than 200 flights on Monday. The central government in Beijing ominously characterized the protest movement as something approaching "terrorism" that posed an "existential threat" to the local citizenry.

A member of airport staff tries to stop anti-government protesters during a demonstration at Hong Kong's international airport. (Issei Kato/Reuters)

At least one Air Canada flight, which was scheduled to fly from Hong Kong to Vancouver, was affected by the Monday closure.

"Customers are being rebooked on a replacement flight, AC2008, departing [Tuesday]," the airline told CBC News in a statement on Monday.

AC2008 is now in flight after a delay, according to Air Canada's website.

Paramilitary police assembling across border

Meanwhile, paramilitary police were assembling across the border in the city of Shenzhen for exercises in what some saw as a threat to increase force brought against the mostly young protesters who have turned out in their thousands over the past 10 weeks.

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 further 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.

The Chinese city of Shenzen is located across the border from Hong Kong. Paramilitary police were assembling in Shenzhen for exercises in what some saw as a threat to increase force brought against the pro-democracy protesters in Hong Kong. (CBC News)

Online images showed armoured personnel carriers belonging to the People's Armed Police driving in convoy Monday toward the location 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.

Watch a segment from the Hong Kong garrison's video:

China's People's Liberation Army in Hong Kong released a video showing footage of 'anti-riot' exercises and featured a red flag with a warning similar to what Hong Kong police have long used during protests. 0:23

The demonstrators have shown no sign of letting up on their campaign to force Hong Kong leader Carrie Lam's administration to respond to their demands, including that she step down and entirely scrap legislation that could have seen criminal suspects sent to mainland China to face torture and unfair or politically-charged trials.

While Beijing tends to define terrorism broadly, extending it especially to nonviolent movements opposing government policies in minority regions such as Tibet and Xinjiang, the government's usage of the term in relation to Hong Kong raised the prospect of the application of heightened violence and the possible suspension of legal rights for those detained.

'Path of no return'

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

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 and police said they arrested another 149 demonstrators over the weekend, bringing the total to more than 700 since early June. Police say several officers have suffered burns, bruises and eye damage inflicted by protesters.

Lam told reporters Tuesday 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," Lam said, "I as the chief executive will be responsible to rebuild Hong Kong's economy ... to help Hong Kong to move on."

Hong Kong's Chief Executive Carrie Lam said at a news conference that dialogue would only begin when the violence stopped. (Thomas Peter/Reuters)

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 Hong Kong.

The protests had until Monday been mostly confined to specific neighbourhoods, police stations and government offices. However, the airport protest 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 for executives travelling across the region.

Hong Kong was promised democratic rights not enjoyed in Communist Party-ruled mainland China when Beijing took over what had been a British colony in 1997, but some have accused Beijing of steadily eroding their freedoms. Those doubts are fuelling the protests, which build on a previous opposition movement that shut down much of the city for seven weeks in 2014 that eventually fizzled out and whose leaders have been imprisoned.