Protesters and police have clashed at Hong Kong's international airport, after hundreds of flights were disrupted for a second day.

The sit-in by pro-democracy protesters took place despite warnings from the city's leader, who said they were heading down a "path of no return".

The new protest came as Beijing sent further ominous signals that the ten 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 this 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.

And late in the evening a small group of riot police briefly used pepper spray to beat back protesters outside the airport as the officers tried to escort a man who was being taken away in an ambulance.

The injured man had earlier been surrounded for more than two hours inside the airport by protesters who were convinced he was an undercover police officer.

During clashes on Sunday, officers disguised themselves as protesters to make arrests, a move which has since sent paranoia soaring about potential infiltrators.

Anger among protesters over what they said were violent tactics deployed by police trying to quell weekend rallies prompted thousands to descend on the airport yesterday.

Airport authorities in response cancelled all flights yesterday afternoon.

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This 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.

Ms 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," she 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 Ms Lam's intransigence for placing it there.

By this afternoon local time, 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.

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

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

Today 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 has voiced concern over police force used against protesters, and called for an impartial probe.