Philadelphia police have said hundreds of looters are ransacking businesses in the city in a second night of unrest after police fatally shot a black man.
Police reinforcements as well as the National Guard have been deployed. Officials say 30 officers were hurt in the first night of clashes.
Police say officers opened fire on Walter Wallace, 27, when he ignored orders to drop a knife he was holding.
Mr Wallace's family say he was suffering a mental health crisis.
What's the situation in Philadelphia?
On Tuesday night, police warned residents to stay away from the riverfront Port Richmond district as widespread looting occurred.
The Philadelphia Office of Emergency Management advised residents across a swathe of the city to stay indoors because of "widespread demonstrations that have turned violent".
According to the Philadelphia Inquirer, protesters tried to erect makeshift barricades using bins. Police used pepper spray and batons after saying they were attacked by demonstrators.
Shops around the city had closed early and constructed barricades on Tuesday.
Residents found pharmacies shuttered and were unable to get medicine, according to CBS News, the BBC's partner in the US.
What have city officials been saying?
Mayor Jim Kenney, a Democrat, has said the video of Monday's fatal police shooting "presents difficult questions that must be answered".
The mayor did not elaborate, but said he was looking forward "to a speedy and transparent resolution for the sake of Mr Wallace, his family, the officers, and for Philadelphia".
Philadelphia Police Commissioner Danielle Outlaw said she had visited the scene and felt the "anger of the community".
"We anticipate the chance of additional incidents of civil unrest," she said at a news conference earlier on Tuesday. "And as such, we will be taking additional steps to ensure order. We will increase our officer presence around the city at key locations."
Large protests broke out in Philadelphia earlier this year following the police killing of George Floyd in Minneapolis, Minnesota. Footage showed white police officer Derek Chauvin kneeling on the black man's neck, while he repeatedly said he was unable to breathe.
Floyd's death sparked protests around the world against racism and police brutality.
What happened on Monday night?
Over 300 people people took to the streets to protest against the shooting of Mr Wallace, and 91 were arrested.
The protests, which continued into the early hours of Tuesday, turned violent at times.
Images from the scene showed a police vehicle set on fire. A number of businesses and shops were looted.
Most of the injured police officers were struck by objects such as bricks and other projectiles, authorities said.
One officer was in hospital with a broken leg and other injuries after being struck by a pickup truck.
What do we know about the shooting?
Police said it happened at about 16:00 (20:00 GMT) on Monday, when two officers responded to a report of a man with a weapon in the neighbourhood of Cobbs Creek in West Philadelphia.
Police spokeswoman Tanya Little told AP news agency that a man, later identified as Mr Wallace, was holding a knife when the officers approached, and instead of following orders to drop the weapon he "advanced towards them".
Both officers fired "several times", hitting Mr Wallace in the shoulder and chest, she said.
One of the officers drove him to a hospital, where he was pronounced dead, according to Ms Little.
Footage shared on social media shows two officers pointing their guns at Mr Wallace as he walks towards them. The officers back away from him and shout at him to put the knife down.
Shots are then fired and Mr Wallace is seen lying on the street.
Mr Wallace's father told the Philadelphia Inquirer that his son had mental health issues and was on medication.
"Why didn't they use a Taser?" he asked.
At Tuesday's press conference, officials said the officers both fired seven shots each. The officers, who have not been named, were wearing body cameras and did not carry stun guns.
Meanwhile, Mr Wallace's family lawyer said that they had called for an ambulance - not police - to help deal with Walter Wallace's mental issues.
Instead, two police officers arrived, lawyer Shaka Johnson was quoted as saying by the Inquirer. Mr Wallace's pregnant wife told them her husband had bipolar disorder and was in crisis.
Reacting to the shooting, Democratic presidential candidate Joe Biden and his running mate Kamala Harris said in a statement: "Our hearts are broken for the family of Walter Wallace Jr, and for all those suffering the emotional weight of learning about another Black life in America lost. We cannot accept that in this country a mental health crisis ends in death."
Philadelphia is the largest city in Pennsylvania, a state critical to next week's presidential election. Source: BBC