When the officer went to question him, the man "immediately" punched him in the face, dazing him, and grabbed for his gun, Walker said.
"The officer is able to move his body away, but the suspect grabs for the gun a second time," Walker said.
Getting control of his gun, the officer fired twice, striking the man in a leg and arm, Walker said.
The officer's name has not been released. He was treated at the Hospital of the University of Pennsylvania and released with a cut to his face.
Police are working to identify the culprit, who gave investigators a false name. He was described as white, in his 30s, and possibly homeless, Walker said.
Police believe the man, who was hospitalized in stable condition, may have been under the influence of narcotics.
"We're still looking to see what his motivation was, who he is, where he's from, and figure out why his actions were what they were," he said.
Philadelphia police will handle the shooting investigation, Walker said, and Drexel University will conduct an internal review of the matter.
The District Attorney's Office will review the joint findings, Walker said.
The officer will be assigned to desk duty until the investigation is complete, said Lori Doyle, a Drexel University spokeswoman.
In an e-mail to students and faculty Saturday, Drexel president John A. Fry said "campus and community safety is Drexel's top priority."
"I am confident that we have a safe campus and neighborhood, and that they will keep getting safer," he said.
Contact Mike Newall at 215-8545-2759 or firstname.lastname@example.org.