The Philadelphia Police Department in recent years has attracted national attention for exhibiting restraint during potentially volatile protests.

The volatility — and pent-up anger — proved overwhelming on Saturday.

After dozens of people gathered outside City Hall and the Philadelphia Museum of Art early in the afternoon to peacefully decry the police killing of George Floyd in Minnesota, the lingering demonstrations morphed into chaos as evening approached, with protesters and others causing hours of turmoil as they moved through Center City.

As the tumult unfolded — with people setting cars on fire, trying to topple the statue of Frank L. Rizzo, breaking windows at City Hall, and ransacking stores across Center City — it was difficult for police to contain the fallout, having to respond to a new incident as soon as they managed to get another one under control.

The protests posed an unenviable challenge for police. Philadelphia’s force — which for decades had a reputation for relying on brutality and coercive tactics — in recent years has consistently sought to avoid confrontation and escalation at protests. The city in 2016 also decriminalized most nuisance offenses to avoid heavy-handed enforcement ahead of the Democratic National Convention.

But with Saturday’s disorder showing no signs of abating, Mayor Jim Kenney took the extraordinary step of declaring a mandatory citywide curfew starting at 8 p.m. Afterward, police were notably more forceful than they had been earlier in the day about taking people into custody.

At a news conference Saturday night, Kenney praised the Police Department for maintaining professionalism while being spit on, insulted, and having projectiles thrown at them.

“I’ve never seen a group of people have more restraint putting up with what they put up with,” Kenney said.

Police Commissioner Danielle Outlaw said 13 officers were injured during the day, as were an unknown number of demonstrators. As of 9 p.m., 14 people had been arrested, police said, without specifying their crimes. Outlaw said additional people were given citations, but she did not specify how many. Four police cars were set on fire.

Kenney vowed that those who broke the law would be held accountable.

Speaker after speaker at Saturday night’s news conference urged those who protested to avoid resorting to destructive behavior — even as they urged people to continuing calling out injustice and push for the improvement of policing.

“Don’t burn down your own house," said City Council President Darrell L. Clarke.

Saturday’s demonstrations began to take a turn about 4 p.m., when a state police vehicle was set on fire on Broad Street near the Vine Street Expressway. Shortly afterward, a large group of people gathered near the Rizzo statue in front of the Municipal Services Building, vandalizing it, trying to tear it down using ropes and chains, and setting a fire beneath it.

At first, police had held a line in front of the building and away from the statue. Over the course of an hour, they inched slowly forward to move the crowd back, with some saying officers had pepper sprayed them.

Officers pushed forward with batons, pepper spray, and riot shields until they were up against the statue itself. The crowds eventually dissipated.

About 5 p.m., people began concentrating nearby at Dilworth Park. Some sprayed graffiti, while others threw makeshift projectiles at the officers, most of whom kept their composure and barely reacted.

Three police cars parked on JFK Boulevard were set on fire, and each was burned down to its chassis. The acrid smoke was so thick at one point that it blotted out the sun.

Later, they moved to the businesses on the plaza, breaking into and destroying them. They took pilfered cans of ginger beer from a cafe and used them as projectiles, shattering some of the lower windows on City Hall.

Another group looted a nearby Starbucks, and threw bags of coffee beans and bagels at the officers, who formed a perimeter around City Hall. Tactical officers eventually arrived, pushing the protesters back using bikes and metal barricades.

But as that was happening, some people began snaking through Center City. Outside the Apple Store on Walnut Street, an officer, visible angry, got into her cruiser and backed into dumpsters that had been pushed toward her car.

All along Chestnut and Walnut Streets, people began looting stores.

Looters hurled heavy stone planters, bricks and skateboards, shattering the windows of stores like Foot Locker, Sephora, Philly Runner and Target, while others dragged newspaper boxes, dumpsters, and the obstacles into the road in anticipation of police cars coming to stop them.

The police presence along those streets was sparse. For roughly two hours, crowds rushed in and out of ransacked stores, running away with shoes, clothes, makeup and other stolen goods. Business owners and residents in nearby apartment towers stood in their lobbies, mouths agape.

As officers began moving through Center City to enforce Kenney’s curfew, many donned helmets and sometimes forced people to the ground. Looters continued breaking into new locations; firefighters responded to buildings that were ablaze.

Civil rights attorney Paul Hetznecker, who organized a team of lawyers to represent people arrested Saturday, said at least 35 were being held at the Ninth District. He was not certain what charges they’d face.

Police declined to say how many additional people had been arrested after curfew.

Late on Saturday, Kenney said the curfew would be in effect Sunday night as well.

Staff writer Anna Orso contributed to this article.