Protesters blocked the Hong Kong airport for a second day. (AFP)

Highlights Pro-democracy protesters blocked Hong Kong airport for a second day

Airport authorities didn't say if flights were still arriving

An earlier statement announced that check-ins had been suspended

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

The new protest came as Beijing also 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 before this week 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.

On Monday a crowd of 5,000 people filled the airport building, the police said. (AFP)

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.

"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.

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, no matter if it's using violence or condoning violence, will push Hong Kong down a path of no return," Lam said.

Protesters IN black T-shirts made barricades using luggage trolleys to prevent passengers from passing through security gates.

"The situation in Hong Kong in the past week has made me very worried that we have reached this dangerous situation."

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.

Airport chaos

But a few hours later the 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 facial injury that reportedly caused a woman to lose the vision in one eye at a demonstration that turned violent on Sunday night.

Demonstrators accused police of causing the injury by firing a bean-bag round.

Some travellers voiced sympathy with the protesters.

Hong Kong leader Carrie Lam made an emotional appeal to protesters to stop violence. (AFP)

"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 told AFP they were angry at having their travel plans thrown into disarray.

"I don't mind what they [the protesters] do but they made us five hours delayed," said 50-year-old Wing Au-yeung, who had stopped off in Hong Kong to collect his aged mother before travelling to South Korea with his family.

"They can do what they want but it should not affect other people."

'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.

The official state news agency Xinhua said in a commentary Tuesday that "violent radicals" were pushing Hong Kong into an "abyss".

In a video posted on Weibo, a CCTV anchor warned viewers: "When dealing with terrorism, there is no soft hand".

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.