Hong Kong airport cancelled all remaining flights on Tuesday after protesters took over the busy international transport hub for the second day in a row.
Demonstrators had earlier prevented some passengers from boarding their flights as hundreds gathered in the terminal.
The new rally 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, has become the biggest challenge to Chinese rule of the semi-autonomous city since its 1997 handover from Britain.
Hong Kong's pro-Beijing leader, Carrie Lam, gave an at-times emotional press conference on Tuesday morning 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.
"The situation in Hong Kong in the past week has made me very worried that we have reached this dangerous situation."
But a few hours after her press conference, thousands of protesters returned to Hong Kong airport with many sitting down in front of security gates at the departure hall.
"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 at the rally.
The protesters set up a long barricade made of luggage trolleys at one of the main security gates, then stood up to block passengers who sought to get through.
On Monday a crowd that police said numbered 5,000 filled Hong Kong's airport to denounce what they said were violent tactics by police in trying to quell weekend rallies.
Hong Kong's airport authorities in response cancelled all flights on Monday afternoon.
Operations resumed on Tuesday morning but hundreds of flights were cancelled even before the fresh afternoon action caused further chaos.
The protesters chanted: "Stand with Hong Kong, stand for freedom," and daubed graffiti that included the term "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.
The demonstrators accused police of causing the injury by firing a bean-bag round.
While the disruptions caused headaches for travellers, many said they sympathised with the protesters.
"The protesters are the loveliest people in the world," said Pete Knox, a 65-year-old Brit on his way to Vietnam.
"I understand the basics of the protest and they've got a point: it's about freedom and democracy and it's incredibly important."
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 to quash them.
The official state news agency Xinhua warned in a commentary Tuesday that "violent radicals" were pushing Hong Kong into an "abyss".
In a video posted on its Weibo channel, 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 rights chief on Tuesday voiced concern over police force used against protesters during massive demonstrations in Hong Kong and called for an impartial probe.
A senior US administration official on Monday urged "all sides" to avoid violence.
"Societies are best served when diverse political views are respected and can be freely and peacefully expressed," the official said on condition of anonymity.