Hundreds of people demonstrated in Philadelphia late Tuesday, with looting and violence breaking out in the 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.

"There's a lot of confusion about why" police shot the young man dead, said Ezra Alidow, a 25-year-old artist on 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 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.

- 'Anger' -

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, in Illinois 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?"

Black woman shot by police to file complaint

An African American woman injured as police shot dead her boyfriend last week announced from her hospital bed Tuesday that she will file a complaint, as she challenged the official version of the shooting.

"I lost the love of my life and the father of my seven-month-old child" when an officer opened fire "while we had our hands raised in the air," said Tafara Williams, 20, during a virtual press conference organized by her lawyer Ben Crump.

America has in recent months undergone a historic reckoning with racism and police brutality, since the killing of African American George Floyd by a white police officer in Minneapolise in May galvanised the largest national protest movement in decades.

Since then, new shootings and incidents of police brutality disproportionately targeting Black people in the US have repeatedly ignited fresh protests.

The shooting of Williams and her boyfriend, 19-year-old Marcellis Stinnette, took place on October 20 in Waukegan, a suburb north of Chicago.

An officer opened fire on the car Williams was driving, injuring her and killing Stinnette, who was in the passenger seat. The officer, who is Hispanic and who has not been named, has since been fired.

Police have said that another officer first tried to investigate what appeared to be a suspicious car, but the couple fled.

The second officer then intervened, but the vehicle allegedly backed towards him and he opened fire.

Williams has said that she was smoking a cigarette in the car, which was parked in front of her home, with Stinette when the first officer approached.

After their conversation she said she left quietly. "I drove off very slowly... The officer was not following me," she said.

But when she turned a corner the second officer appeared to be waiting. 

Through tears, she said there was "a crash."

"I lost control, the officer was shooting at us... I kept shouting, I don't have a gun," she said.

Her hands were up, she said, but she could not move due to her injuries. 

"I kept asking why, why he was shooting."

The city mayor has promised to make video of the shooting public this week, and the families have called for calm.

Crump, who is representing Williams and her family, praised the initial steps taken by police but said he will file a civil suit for compensation and police reform.

"When you're dealing with African Americans, it's almost as if you shoot first ask questions later," he said.