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."
In a statement released early Wednesday the White House said it "stands ready, upon request, to deploy any and all Federal resources to end these riots."
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.
"There's a lot of confusion about why" police shot the young man dead, Ezra Alidow, a 25-year-old artist, said Tuesday.
"It's happening all over America. It's scary," he said. "These police were undertrained."
More than 90 arrests were made during the first night of sporadic riots and looting in the city Monday, and 30 police officers were injured, including one whose leg was broken when he was hit by a truck.
"For today, and this evening, we anticipate the chance of additional incidents of civil unrest," Philadelphia Police Commissioner Danielle Outlaw told reporters.
"As such we will be taking additional steps to ensure order," including increasing police presence at key locations and deploying the looting response team, she added.
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 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.
The White House statement said the disturbances were "the most recent consequence of the Liberal Democrats' war against the police."
The Democratic challenger and his running mate Kamala Harris said in a statement that their "hearts are broken" for Wallace's family.
But they also called on demonstrators to protest peacefully.
"No amount of anger at the very real injustices in our society excuses violence," they said.
"Looting is not a protest, it is a crime. It draws attention away from the real tragedy of a life cut short," Biden and Harris added.
Local media reported that two officers shot Wallace around 4:00 PM (2000 GMT) on Monday afternoon after he refused to drop the knife as his mother tried to restrain him.
Phone video of the killing posted on social media showed Wallace push his mother away and then walk towards the police.
"Put the knife down," one of the officers shouted in the video, which panned away as officers opened fire.
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.
Outlaw launched an investigation, saying the video "raises many questions."
"While at the scene this evening, I heard and felt the anger of the community," she said in a statement.
The shooting of Wallace comes a week after an officer in Waukean, north of Chicago, shot dead 19-year-old Black man Marcellis Stinnette when he opened fire on his vehicle.
His 20-year-old partner, Tafara Williams, was also wounded.
"When does it end America?" asked civil rights lawyer Ben Crump at a press conference Tuesday.
"How many more Black people have to be killed because of police brutality, excessive force, bias, systematic racism, deliberate indifference?"