HONG KONG -- Embattled Hong Kong leader Carrie Lam said Tuesday that she has received the backing of Chinese President Xi Jinping in her handling of five months of anti-government protests in the semi-autonomous Chinese territory, as hundreds of masked demonstrators took to the streets again.

Xi and Lam unexpectedly held talks Monday night on the sidelines of a trade event in Shanghai amid signals from China's central government that it may tighten its grip on Hong Kong to quell the unrest that has at times challenged Chinese rule.

Lam told a news conference in Shanghai that Xi expressed "care and concern" during their brief meeting, along with support for measures taken by her government to end the crisis. She vowed that the government will strive to stamp out violence with strict law enforcement.

Lam said she was disturbed by mounting injuries during the protests, including an incident early Monday that left a 22-year-old university student sprawled in a pool of blood at a carpark building after police fired tear gas. Hospital officials said the victim was in a critical condition.

Lam said investigations would be carried out to determine exactly what happened, and that the case drove home the government's message that violence must cease.

Television footage showed riot police firing tear gas at the building after objects were hurled down at the street at them when they chased off a mob. Minutes later, medical workers found the unconscious student on the second floor of the building.

Senior police official Suzette Foo said late Tuesday that the young man had reportedly fallen from an upper floor, but that it wasn't captured by security surveillance cameras. She didn't rule out the possibility that he was fleeing from tear gas but noted that police fired from a far distance. She also rebutted online claims that police pushed the victim down.

"It is an upsetting incident. We will certainly investigate this case fully and do all we can to find out the truth," Foo said.

Hundreds of black-clad demonstrators wearing Guy Fawkes masks -- which are protest symbols worldwide -- rallied Tuesday night in Hong Kong's busy Tsim Sha Tsui district to mark the one-month anniversary of a government ban on facial coverings at rallies. Some protesters vandalized shops and set up road barriers as they marched along streets.

"We are out to tell the government that we will not be cowed. We will fight to the end, Hong Kong people will not give up," a protester wearing a Guy Fawkes mask said on local television. Police later fired a water cannon and quickly broke up the rally.

Earlier Tuesday, Hong Kong Chief Secretary Matthew Cheung said Xi's meeting with Lam was a "vote of confidence" in the city's government. But pro-democracy lawmaker Claudia Mo warned of a tougher stance by Beijing.

"The message to Hong Kong people is that we are with her, she has our backing and you better watch out," Mo told The Associated Press.

China's official Xinhua News Agency reported that Xi also "demanded unswerving efforts to stop and punish violent activities." He called for more dialogue and efforts to improve people's livelihoods in one of the world's most expensive cities.

China's Communist Party last week indicated it may try to find a way to enact anti-subversion laws in Hong Kong, after such measures were shelved previously due to public opposition.

The protests began in early June against an extradition bill that would have allowed suspects to be sent for trials in mainland China, which many saw as infringing on Hong Kong's judicial freedoms and other rights that were guaranteed when the former British colony return to China in 1997.

Lam abandoned the bill three months into the protests, but the movement by then had grown into calls for greater democracy and police accountability and had become one of Xi's biggest challenges since he came to power in 2012. Lam provoked further anger by invoking emergency powers to ban masks at rallies.

More than 3,300 people have been arrested since the protests began. In a bloody incident Sunday night, a knife-wielding man believed to be a Beijing supporter slashed two people after an argument and bit off part of a local politician's ear outside a mall. Police have arrested the assailant and two men who attacked him.

Cheung said the government plans to hold a second community dialogue after Nov. 24 district elections. Lam held her first town hall meeting on Sept. 26, where she was criticized by angry residents.

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Associated Press researcher Shanshan Wang in Beijing contributed to this report.