WATCH LIVE: Chaos in Hong Kong airport as police clash with pro-democracy demonstrators

Hundreds of flights were cancelled or suspended at Hong Kong's airport Tuesday as pro-democracy protesters staged a second disruptive sit-in at the sprawling complex, defying warnings from the city's leader who said they were heading down a "path of no return".

Watch live coverage, courtesy of The Washington Post, below:

The new protest came as Beijing sent further ominous signals that the 10 weeks of unrest must end, with state-run media showing videos of security forces gathering across the border.

The crisis, which has seen millions of people take to Hong Kong's streets, was already the biggest challenge to Chinese rule of the semi-autonomous city since its 1997 handover from Britain.

But the two days of protests at the airport, one of the busiest in the world, raised the stakes yet again.

All check-ins were cancelled on Tuesday afternoon after thousands of protesters wearing their signature black T-shirts made barricades using luggage trolleys to prevent passengers from passing through security gates.

Scuffles broke out between protesters and travellers who pleaded to be allowed past.

"I want to shut down the airport just like yesterday so most of the departure flights will be cancelled," a 21-year-old student, who gave his surname as Kwok, told AFP.

On Monday a crowd that police said numbered 5,000 filled the building to denounce what they said were violent tactics by police in trying to quell weekend rallies.

Airport authorities in response cancelled all flights on Monday afternoon.

- 'Path of no return' -

On Tuesday morning, the city's leader, Carrie Lam, gave an at-times emotional press conference in which she warned of dangerous consequences if escalating violence was not curbed.

"Violence... will push Hong Kong down a path of no return," she said.

Lam, who faced fierce questioning from local reporters and at one point appeared to be on the verge of tears, appealed for calm.

"Take a minute to think, look at our city, our home, do you all really want to see it pushed into an abyss," Lam said, although she again refused to make any concessions to the protesters.

In an interview with the BBC, Hong Kong's last colonial governor Chris Patten agreed the city was "close to the abyss" but blamed Lam's intransigence for placing it there.

By the Monday afternoon, protesters returned to the airport chanting "Stand with Hong Kong, stand for freedom," and daubing graffiti that included "an eye for an eye".

This was in reference to a serious injury reportedly suffered by a woman and caused by a bean-bag round fired during a demonstration that turned violent on Sunday night.

Some travellers voiced sympathy with the protesters.

"I understand the basics of the protest and they've got a point: it's about freedom and democracy and it's incredibly important," said Pete Knox, a 65-year-old Briton on his way to Vietnam.

Others were conflicted.

"I do really feel for the protesters here, I really do. But I can't quite reconcile with myself whether this is the right way of doing it," said Chun-sun Chan, 46, who was trying to fly home to his two children in Britain.

As the day wore on there were multiple confrontations as visitors pleaded in vain to be allowed through the blockade.

"I have a baby, I need to get home to my children," a blonde woman could be heard saying through tears on one video posted by reporters online.

- 'Mobsters' -

The protests began in opposition to a bill that would have allowed extraditions to the mainland, but quickly evolved into a broader bid to reverse a slide of rights and freedoms in the southern Chinese city.

Authorities in Beijing on Monday slammed violent protesters who threw petrol bombs at police officers, linking them to "terrorism".

On Tuesday state media upped the ante, calling protesters "mobsters", warning they must never be appeased and raising the spectre of mainland security forces intervening.

Meanwhile, videos promoted by state media showed Chinese military and armoured vehicles appearing to gather in the southern city of Shenzhen, which borders Hong Kong.

The UN's rights chief on Tuesday voiced concern over police force used against protesters, and called for an impartial probe.

A senior US administration official on Monday urged "all sides" to avoid violence.