Protesters clashed with the police after the death of Walter Wallace Jr., who authorities said had a knife.
Hundreds of people demonstrated in Philadelphia late Tuesday, with looting and violence breaking out in a second night of unrest after the latest police shooting of a Black man in the US.
The police department warned on Twitter that "a large crowd" of around 1,000 people was looting businesses in the area of Castor and Aramingo, advising citizens to "avoid the area."
Footage from a news helicopter appeared to show people breaking into and looting a Foot Locker store and another business.
An AFP reporter at a different location, in West Philadelphia where another crowd of an estimated 1,000 people had gathered, saw police armed with batons clashing violently with several dozen protesters.
Pennsylvania Governor Tom Wolf's office announced the deployment of several hundred National Guard troops to the city "to protect the right to peacefully assemble and protest while keeping people safe."
The fresh unrest came a day after the death of 27-year-old Walter Wallace, whose family said he suffered mental health issues.
On Monday night hundreds of demonstrators took to the streets, with riot police pushing them back with shields and batons.
During a night of sporadic riots and looting in the city, around 30 police officers were injured, including one whose leg was broken when hit by a truck.
A spokesman for Philadelphia police said most injuries were caused by projectiles such as bricks and rocks.
"All (are) in stable condition at various hospitals," police spokesman Miguel Torres said.
The US has seen a wave of protests and rioting since the police killing of George Floyd in May in Minnesota, when an officer was filmed pressing his knee to handcuffed Mr Floyd's neck until he went limp.
Many of the protests have accused the police of racism and brutality, but President Donald Trump has focused on the unrest to bolster his claims to be the "law-and-order" candidate in his election battle against Joe Biden.
"We'll be standing by and prepared to deploy federal resources, if necessary," White House director of communications Alyssa Farah told Fox News on Tuesday.
"President Trump will not tolerate any violence directed at America's law enforcement. We're going to let the investigation play out, but we will not tolerate lawlessness in our streets."
Local media said phone video of Monday's killing posted on social media showed Mr Wallace push his mother away and then walk toward the police.
"Put the knife down," one of the officers shouted in the video, which panned away as officers opened fire.
Mr Wallace's father, also called Walter Wallace, said his son appeared to have been shot 10 times, according to the Philadelphia Inquirer.
"“Why didn't they use a Taser?" the paper quoted him as saying. "He has mental issues. Why you have to gun him down?" he added, saying that his son was on medication.