* Evening clashes at airport between protesters and police
* U.N. urges HK to exercise restraint as protests intensify
* Airport suspends check-in as demonstrations grow
* Stock market drops, leader Carrie Lam defiant at news briefing
* Protests pose major challenge to Hong Kong and Chinese rule
By Tom Westbrook and Clare Jim
HONG KONG, Aug 13 (Reuters) - Protesters clashed with police at Hong Kong's international airport on Tuesday evening after flights were disrupted for a second day, plunging the former British colony deeper into turmoil.
The scuffles broke out in the evening between police and protesters, after an injured person was taken out of the main terminal by medics.
Several police vehicles were blocked by protesters, and riot police moved in, pushing some protesters back and using pepper spray at times amid heated scenes.
Protesters also barricaded some passageways in the airport with luggage trolleys and other objects.
Hong Kong's Airport Authority said operations at the airport had been "seriously disrupted" and that departing passengers had been unable to reach immigration counters.
Hong Kong's stockmarket fell to a seven-month low.
Ten weeks of increasingly violent clashes between police and protesters have roiled the Asian financial hub as thousands of residents chafe at a perceived erosion of freedoms and autonomy under Chinese rule.
The United Nations High Commissioner for Human Rights urged Hong Kong to exercise restraint and investigate evidence of its forces firing tear gas at protesters in ways banned under international law.
"Take a minute to look at our city, our home," Chief Executive Carrie Lam said, her voice cracking, at a news conference in the government headquarters complex, which is fortified behind 6-foot (1.8-m) -high water-filled barricades.
"Can we bear to push it into the abyss and see it smashed to pieces?"
China this week condemned some protesters for using dangerous tools to attack police, calling the clashes "sprouts of terrorism". They present President Xi Jinping with one of his biggest challenges since he came to power in 2012.
Hong Kong legal experts say Beijing might be paving the way to use anti-terrorism laws to try to quell the demonstrations.
The clashes at the airport followed an unprecedented airport shutdown on Monday.
Again on Tuesday, thousands of black-clad protesters jammed the terminal, chanting, singing and waving banners.
Floors and walls were covered with missives penned by activists and other artwork. Initially, the scene was peaceful as knots of protesters spoke to travellers, explaining their aims.
"Sorry for the inconvenience, we are fighting for the future of our home," read one protest banner at the airport.
"I think paralysing the airport will be effective in forcing Carrie Lam to respond to us ... it can further pressure Hong Kong's economy," said Dorothy Cheng, 17.
'PANIC AND CHAOS'
The weeks of protests began as opposition to a now-suspended bill that would have allowed suspects to be extradited to mainland China, but have swelled into wider calls for democracy.
Demonstrators say they are fighting the erosion of the "one country, two systems" arrangement that enshrined some autonomy for Hong Kong since China took it back from Britain in 1997.
They want Lam to resign. She says she will stay.
"My responsibility goes beyond this particular range of protest," Lam said on Tuesday, adding that violence had pushed the territory into a state of "panic and chaos".
As she spoke, the benchmark Hang Seng index hit a seven-month low. It shed more than 2%, dragging down markets across Asia.
Lam did not respond to questions at a press briefing to clarify if she had the power to withdraw the extradition bill and satisfy a key demand made by the protesters, or if she needed Beijing's approval.
Airport authorities had earlier suspended check-in operations. Crowds of protesters continued to swell in the evening.
"Terminal operations at Hong Kong International Airport have been seriously disrupted as a result of the public assembly," the airport authority said.
Some passengers challenged protesters over the delays as tempers began to fray, while the demonstrators, using a Chinese term of encouragement, chanted, "Hong Kong people - add oil!"
Flag carrier Cathay Pacific said: "There is potential for further flight disruptions at short notice".
The airline, whose British heritage makes it a symbol of Hong Kong's colonial past, is also in a political bind.
China's civil aviation regulator demanded that the airline suspend staff who joined or backed the protests from flights in its airspace, pushing the carrier's shares past Monday's 10-year low.
Other Chinese airlines have offered passengers wanting to avoid Hong Kong a free switch to nearby destinations, such as Guangzhou, Macau, Shenzhen or Zhuhai, with the disruption sending shares in Shenzhen Airport Co Ltd surging. (Additional reporting by Felix Tam, Noah Sin, Donny Kwok, Greg Torode and James Pomfret in Hong Kong Additional reporting by Jamie Freed in Singapore and Stella Qiu in Beijing Writing by Tom Westbrook and James Pomfret Editing by Clarence Fernandez and Frances Kerry)
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