Hong Kong leader Carrie Lam says China could step in with military intervention to prevent further violent protests if local authorities are unable to stop them.

Key points: The emergency laws had not been invoked since 1967

The emergency laws had not been invoked since 1967 Teenagers were shot and wounded in the protests

Teenagers were shot and wounded in the protests Hong Kong leader Carrie Lam has urged young people not to engage in political activities

Ms Lam has, however, ruled out using colonial-era emergency powers to introduce further new laws after a long weekend of protests saw widespread defiance from demonstrators over the ban on face masks that she instituted.

Ms Lam made the comments as Hong Kong braced for further protests this week.

Those protests had previously turned ugly as authorities and demonstrators clashed — with the altercation at the weekend marking the first clash between protesters and Chinese troops stationed on the island.

Ms Lam was adamant Hong Kong did not need further Chinese assistance to deal with the protesters.

"I cannot tell you categorically now under what circumstances we will do extra things, including … calling on the Central Government to help," Ms Lam said.

On Friday, Ms Lam invoked the emergency powers for the first time in more than 50 years to outlaw face masks, which protesters have used to shield their identities. Those powers had not been used since 1967 when there was a pro-communist uprising against the British.

But the move inflamed demonstrations over the weekend and thousands took to the streets wearing masks in defiance.

Hong Kong leader Carrie Lam has suggested there will be no more use of special powers. ( AP: Vincent Thian )

Ms Lam said it was too early to say whether the mask ban would be ineffective and urged young people not to participate in what she called political actions.

Rallies deteriorated into running clashes, with police firing tear gas and charging with batons to disperse protesters across the city.

Two teenage protesters were shot and wounded, one in the chest and the other in the leg, during skirmishes with police in some of the recent violence.

ATMs, Chinese banks and scores of shops were vandalised during protests over the long weekend. Many restaurants and malls closed early over what was typically a very busy holiday period.

Hong Kong's metro, which carries about 5 million passengers a day, said on Tuesday some stations would not open for service because damaged facilities needed to be repaired. Train services would also end at 8:00pm, more than four hours earlier than normal.

Facial masks have been banned by Hong Kong's chief executive using emergency powers. ( Etan Liam/flickr.com/CC BY-ND 2.0 )

Speaking at a weekly news conference, Ms Lam said tourist numbers had fallen sharply and the impact on the city's third-quarter economic data would "surely be very bad".

She appealed to property developers and landlords to offer relief to retailers whose businesses had been hit.

"For the first six days of October, during the so-called Golden Week holiday, the numbers visiting Hong Kong plunged more than 50 per cent," she said.

US weighs in

The protests, which show no sign of abating, pose the biggest challenge to Chinese President Xi Jinping since he came to power in 2012 and are Hong Kong's thorniest political crisis since Britain returned the then Crown colony to China in 1997.

What started as opposition to a now-withdrawn extradition bill has grown into a pro-democracy movement against what is seen as Beijing's increasing grip on the city, which protesters say undermines a "one country, two systems" formula promised when Hong Kong returned to Chinese rule.

China has dismissed those accusations, saying foreign governments, including Britain and the United States, have fanned anti-China sentiment.

US President Donald Trump said on Monday that if anything bad happened in Hong Kong it would be bad for the US-China trade talks.

An increasing number of US politicians voiced anger on Monday over the National Basketball Association's response to a Houston Rockets official's tweet backing the protests.

Reuters