Philadelphia, Oct 29 A city-wide curfew has been imposed in Philadelphia after two consecutive nights of violent protests against the police killing of an African-American man, according to media reports.

The curfew, which started at 9 p.m. on Wednesday, will continue until 6 a.m., on Thursday morning, reports Fox News.

The protests which took place on Tuesday night and into Wednesday morning, were triggered after Walter Wallace Jr. was fatally shot by two police officers.

A video footage shows the 27-year-old victim, armed with a knife, walking on a street in West Philadelphia.

As people yelled, two police officers aimed their guns at him.

He eventually fell to the ground after at least seven rounds of gunfire.

The protests were marred with violence, including the shooting of 15 people and ransacking of multiple businesses in the Port Richmond neighbourhood.

According to authorities, at least 81 people have been arrested on various charges, while 23 police officers were injured.

On Thursday, William McSwain, the US Attorney for the Eastern District of Pennsylvania, will announce criminal indictments related to the unrest, Fox News reported.

After signing a disaster proclamation, Pennsylvania's Democrat Governor Tom Wolf tweeted late Wednesday night: "Hundreds have gathered peacefully to speak out against social injustice. But their voices are being drowned out by others taking advantage of this time to sow mayhem.

"I signed this proclamation so commonwealth resources can be provided quickly to protect lives and property."

According to Police Commissioner Danielle Outlaw, a probe into Wallace Jr's killing was underway.

Wallace had struggled with mental health issues and was bipolar, a family member previously told Fox News.

The US has witnessed massive protests this year over the killing of African-Americans by police officers.

George Floyd, an unarmed African-American man, died in Minneapolis on May 25 after a white police officer knelt on his neck for nearly nine minutes.

In a similar incident on August 23, Jacob Blake was shot in the back seven times by a white policeman in the city of Kenosha, Wisconsin.

On March 23, Daniel Prude died in Rochester, New York after the police held him down on the road for two minutes and also put a "spit hood" on his head.

Although Prude died in March, the incident only came to light in September.

Meanwhile, Breonna Taylor, a 26-year-old hospital worker, died after she was was shot multiple times as officers stormed her home on March 13.

Ahmaud Arbery was chased, shot and killed while jogging outside Brunswick, Georgia on February 23.

Philadelphia imposes curfew amid protests over police killing