After a night of rioting, Lam calls on Hong Kong residents to condemn violence

"I urge you to ... be determined in severing ties with rioters," she said.

HONG KONG -- A day after violent protests rocked Hong Kong, its embattled leader called on citizens Saturday to condemn violence and reject the rioters.

In a video statement released Saturday afternoon local time, Chief Executive Carrie Lam slammed Friday night's violence, which was prompted in part by a ban on face masks that Lam had announced earlier Friday.

"The extreme behavior of rioters caused a very dark night in Hong Kong, semi-paralyzing Hong Kong society, making many people very worried and scared,” she said. “The Hong Kong SAR (the official special administrative region government) is determined to stop violence. I urge you to support the SAR government to stop violence in accordance with the law, condemn violence, and be determined in severing ties with rioters."

Lam on Friday used a colonial-era emergency law to ban protesters from wearing face masks after four months of anti-government demonstrations that have, at times, turned violent and put the city on edge. She said she decided to invoke the Emergency Regulations Ordinance, which dates back to 1922 when Hong Kong was under British colonial rule and gives the city's leader sweeping powers.

The ban came into force at midnight local time, but not before numerous restaurants and stores that protesters believed were pro-China were trashed, including Starbucks, because they partnered with local pro-China partner Maxim's. Multiple Hong Kong subway stations were completely vandalized, and entrances were set on fire leading to the entire system shutting down for the first time in history.

An off-duty police officer in Yuen Long was surrounded and attacked by protesters. Video has emerged of the assault, showing the office being attacked by protesters as he emerged from his car. The officer fell to the ground and was beaten by the group, authorities said. In self-defense, the officer fired one shot. Then a rioter threw a petrol bomb at the officer and set him on fire. In the process, he dropped his gun but was able to retrieve it before protesters could grab it. While waiting for reinforcement, another petrol bomb was thrown at the officer and his feet were on fire.

By Saturday afternoon, some demonstrators, many still with masks, had returned to the streets. Hong Kong's subway system was still closed down, and 20 malls as well as a major supermarket chain were shuttered.

On Friday Lam said the escalation in violence during the protests "has reached a very alarming level in the past few days, causing numerous injuries and leading Hong Kong to a chaotic and panic situation."

"We are particularly concerned that many students are participating in these violent protests or even riots, jeopardizing their safety and even their future," she added. "As a responsible government, we have the duty to use all available means in order to stop the escalating violence and restore calmness in society."

Of the ban on masks, Lam said, "We believe the new law will create a deterrent effect against masked protesters and rioters. We are now in a rather extensive and serious public danger. It is essential for us to stop violence and restore calm to society as soon as possible."

The anti-mask law targets "rioters or those who resort to violence," Lam said, and it contains exemptions for those who have legitimate needs or reasons to cover their face, such as for health, religious or professional reasons.

Lam also noted that the ban is "subsidiary legislation," and thus it will be debated by Hong Kong's Legislative Council when they resume meetings later this month.

The announcement outraged many protesters who vowed to defy the new law with a full-mask march when it took effect Saturday. Thousands of masked demonstrators took to the streets even before Lam spoke at the press conference.

Face masks have been a distinct feature of the protesters in Hong Kong, a semi-autonomous Chinese territory. The movement began in early June when hundreds of thousands of mostly young people marched against a proposed extradition bill that would have allowed suspected criminals in Hong Kong to be sent to mainland China for trial.

Lam has since withdrawn the controversial bill, but widespread unrest has continued as demonstrators broaden their demands to include a call for direct elections for the city's leaders, amnesty for protesters and an independent investigation into alleged police brutality.

ABC News' Marc Nathanson contributed to this report.