Philadelphia has been the scene of protests as well as riots and looting since Monday following the police shooting of a Black man, Walter Wallace Jr.
A footage, filmed by a bystander, shows police officers have fired 14 rounds at him after he ignored their commands to drop a knife.
Wallace, 27, was said to be suffering from mental health episode at the time of his death.
“The mayor or whoever it is that's allowing people to riot and loot and not stop them is also just a horrible thing,” Trump said on Wednesday.
“I saw the event. Everybody did, it was on television. It was a terrible event. I guess that's being looked at very strongly. We have the federal government looking at it also. But the rioting in Philadelphia has to stop. They have to stop it,” he added.
The Republican president also asked the Democrats in the city to crack down on the rioters, reiterating that Republican-run cities do not face such issues.
“They should bring in the National Guard if the police need help and they should do something. You can't let that go again. A Democrat-run state, a Democrat-run city,” Trump noted.
He further claimed that the group of people who are behind the unrest are favoring the support of Joe Biden.
“This is a group that he supports. He doesn't want to condemn them," Trump said. "I saw that he doesn't want to condemn them. You have to condemn, you have to be strong. You can't have chaos like that. And he'll be very, very weak."
Trump's remarks come hours after Biden addressed the violence in Philadelphia, telling reporters that the death is not a good excuse for looting or criminal activity.