The teenager has been struck by a bullet after an 18-year-old became the first protester to be hit by a live round on Tuesday.
Thousands of pro-democracy protesters in Hong Kong have defied a ban on face masks on another night of chaos in the territory.
The entire metro network was closed down by authorities amid widespread overnight violence as activists set fires, attacked stations, looted and engaged in running battles with police.
A teenage boy was shot in the leg by police and is in a serious condition, according to Sky sources.
Protesters wearing masks fill the main roads of Hong Kong after the government announced a face mask ban
— Benjamin Franklin ? (@benFranklin2018) October 4, 2019
It is unclear whether the 14-year-old was shot with a live round or a rubber bullet, and whether it was fired by the police or someone else.
After seeing her government’s ban on protesters wearing masks in public ignored, beleaguered chief executive Carrie Lam said on Saturday her administration could “no longer tolerate rioters destroying Hong Kong”.
Her ban, introduced under emergency powers used for the first time in more than 50 years, did not discourage demonstrators, who took the streets in the sort of numbers that have become a familiar since protests began in June.
The closure of the entire MTR network, which handles more than four million trips a day, is a major inconvenience for the territory.
“From MTR to EmptyR,” tweeted leading activist Joshua Wong.
Queues also formed outside banks, many of which closed, some after being attacked by protesters.
From MTR to EmptyR pic.twitter.com/3Zvv9MLRXv
— Joshua Wong 黃之鋒 (@joshuawongcf) October 5, 2019
The boy, who was shot in the left thigh, is at Tuen Mun Hospital.
On Tuesday, China’s National Day, Tsang Chi-kin, 18, became the first person to be shot with a live round since the protests started.
He was hit in the shoulder and was later charged with rioting, an offence which carries a penalty of up to 10 years in jail.
Video showed a police officer in the Yuen Long district, reportedly off duty, being beaten by protesters and having a petrol bomb thrown at him, which exploded, briefly setting fire to his foot.
As he sought to escape the flames, he dropped his gun and had to grab it before a demonstrator picked it up.
Other footage emerged of rioters smashing ticket gates, setting off sprinklers at the Sha Tin Metro station.
Earlier, Ms Lam said Hong Kong faced “extensive and very serious danger” but stressed it was not under a state of emergency.
“We must save Hong Kong the present Hong Kong and the future Hong Kong,” she said. “We must stop the violence. We can’t just leave the situation to get worse and worse.”
Anyone breaking the mask ban could be jailed for a year or fined.
It came into force at midnight in the territory, which is seven hours ahead of the UK.
One protester said: “Will they arrest 100,000 people on the street? The government is trying to intimidate us, but at this moment, I don’t think the people will be scared.”
Another protester noted that Hong Kong police wear masks “and they don’t show their pass and their number”.
“So, I will still keep my mask on everywhere,” he said.
Face masks have become a hallmark of protesters in Hong Kong, even at peaceful marches, amid fears of retribution at work or of being denied access to schooling, public housing and other government-funded services.
Many are concerned their identities could be shared with the massive state-security apparatus that helps keep the Communist Party in power in mainland China, where high-tech surveillance including facial recognition technology is ubiquitous.