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 U.S.

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."

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," said Philadelphia Police Commissioner Danielle Outlaw, adding that "As such we will be taking additional steps to ensure order," including increasing police presence at key locations and deploying the looting response team.

The U.S. 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 protesters 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 p.m. local time 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.

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.

(With input from AFP)