Lam urged foreign critics to accept that the four months of protests marked by escalating violence were no longer "a peaceful movement for democracy."

She said seeking Chinese intervention was provided for under Hong Kong's constitution but that she cannot reveal under what circumstances she would do so.

"I still strongly feel that we should find the solutions ourselves. That is also the position of the central government, that Hong Kong should tackle the problem on her own, but if the situation becomes so bad, then no options could be ruled out if we want Hong Kong to at least have another chance," she said at a news conference.

The protests started in June over a now-shelved extradition bill that would have allowed some criminal suspects to be sent to mainland China for trial but have since morphed into a larger anti-government movement. Protesters say the bill is an example of Beijing's increasing influence over the former British colony, which was promised a high level of autonomy when it returned to Chinese rule in 1997.

The unrest has pummeled tourism and hurt businesses in the global financial hub, further bruising the city's economy as it grapples with the effects of the U.S.-China trade war.

U.S. President Donald Trump on Monday urged Chinese President Xi Jinping to ensure a "humane solution" in Hong Kong. He warned that any "bad" outcome could hurt trade talks ahead of negotiations in Washington on Thursday.

Hardening her government's stance on the protests last week, Lam invoked a colonial-era emergency law to criminalize the wearing of masks at rallies but it fueled more anger, with daily violence over the long holiday weekend. Police officers last week fired gunshots while under attack from protesters, wounding two teenagers who were the first victims of police gunfire since the protests started.

Enforcement of the mask ban began Saturday, and Lam said it was too early to call it a failure.

Police regional chief Kwok Yam-yung said 241 people were detained in widespread "atrocities" over the last four days that saw ferocious attacks on officers and those with opposing views.

He said that included 77 who violated the mask ban and that 14 of them were charged Tuesday, bringing the total number of prosecutions to 16. The mask ban is punishable by up to a year in jail and a fine, which is light in comparison to those accused of rioting, which carries a penalty of up to 10 years.

Since June, 2,363 people have been arrested, with more than 200 charged with rioting.

"Ruthless and reckless acts are pushing the rule of law to the brink of total collapse," Kwok said.

Critics fear the emergency law, which gives Lam broad powers to implement any measures she deems necessary, could pave the way for more draconian moves. Lam said the government would make a "careful assessment" before deciding on other emergency measures such as internet controls.

She also pledged to continue a dialogue and take steps to address livelihood and economic problems in a policy address due Oct. 16 when the Legislative Council resumes.

Protesters stormed and damaged the legislative building on July 1, requiring repairs. Lam appealed for peace when the legislative session reopens, warning that further disruptions would set back the approval of bills and impede the city's development.

The city's subway and trains, which carry some 5 million passengers daily, mostly reopened Tuesday but will shut early amid fears of more protests. The entire MTR network was shut down Saturday, with limited service the last two days.

Videos on local media showed masked protesters smashing windows of a train heading to mainland China late Monday as passengers screamed — the first time a train carriage was attacked. Protesters also threw objects on the track as the train pulled away. An MTR spokesman, who identified himself only as Terry, confirmed the incident and said some cross-border services were suspended Tuesday.

Scores of students wore masks in defiance as they returned to school after the holiday. Some rallied at lunchtime, chanting slogans and holding placards that read "You may take away my mask but not my belief" and "Ideas are bullet-proof."