A woman reacts while being arrested during a protest of the death of George Floyd on May 31, 2020 in Philadelphia, Pennsylvania. Photo / Getty Images
Footage released on social media shows violent protesters in Philadelphia destroying rows of police cars and taking what they can out of them, in what's been described as "complete anarchy".
Several videos show police vehicles ablaze or with smashed windows, and protesters looting stores in the city.
Philadelphia is imposing a curfew from 6pm today, local time, in an attempt to curb the violence.
US President Donald Trump took to Twitter to urge local Philadelphia leaders to call in the National Guard.
"Law & Order in Philadelphia, NOW! They are looting stores. Call in our great National Guard like they FINALLY did (thank you President Trump) last night in Minneapolis. Is this what voters want with Sleepy Joe? All Dems," he tweeted.
Protests have also turned violent in Brooklyn, New York, with rioters taking over police cars and setting them on fire.
Curfews will continue in cities across the country as municipalities brace for further protests and unrest.
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Some four million people are in the firing line.