The lockdown will start at 9pm on Wednesday and run until 6am on Thursday, mayor Jim Kenney and other officials has told a briefing.

Tension has gripped Philadelphia since Monday's deadly police shooting of Walter Wallace, 27, who was armed with a knife and described by relatives as suffering a mental breakdown.

Police have made 172 arrests and 53 officers have been injured in two nights of protests marked by significant looting of big-box stores and other businesses, some still recovering from earlier unrest.

City officials said as many as 1000 people were involved in looting in one corner of the city on Tuesday night, catching police off guard.

"These individuals are doing nothing but simply wasting our precious resources," Police Commissioner Danielle Outlaw said, lamenting what she described as "widespread lawlessness" in Pennsylvania's biggest city.

The unrest has turned Philadelphia into the latest flashpoint in the United States on issues of race and police use of force just days ahead of Tuesday's presidential election.

Donald Trump, who is seeking a second term in office, has made policing a top campaign issue, calling for a tough "law and order" stance on the protests.

In Nevada on Wednesday, he said the events in Philadelphia were "terrible" and offered to send federal help.

"What I'm witnessing is terrible and frankly that the mayor or whoever it is that's allowing people to riot and loot and not stopping them is also just a horrible thing," he said.

Pennsylvania is a crucial battleground state in the race between Trump and Democratic challenger Joe Biden, who has said he supports police but wants to address racial inequalities.

Pennsylvania Governor Tom Wolf, a Democrat, said on Wednesday his administration stood ready to help the city including to "avoid unnecessary harm to businesses that provide services to affected neighbourhoods".

"We must address the systemic problems that cause tragedies against people of colour," he said on Twitter.

On Tuesday, Wolf ordered several hundred National Guard officers to prepare to assist.

Mayor Kenney said they would be brought in to "safeguard property and prevent looting" without providing a time frame.

Tuesday's rallies began peacefully but grew confrontational as darkness fell, just as on the previous day.

Police turned out in force to cordon off a West Philadelphia commercial district that was looted the previous night.

But looters broke into businesses elsewhere, including in the city's Port Richmond section, video from WPVI television showed. At times, police in riot gear shoved protesters back from barricade lines.

The violence erupted despite the pleading of the dead man's father, Walter Wallace Sr, who on Tuesday urged people to "stop the violence" out of respect for his son and family.

"I don't condone no violence, tearing up the city, looting of the stores and all this chaos," he told reporters and a gathering of people.

"It's an SOS to help, not to hurt."

Australian Associated Press