The assault happened around 2 a.m. Friday near Rittenhouse Square Park at Walnut Street and South 18th Street.
According to police, the 27-year-old victim left a bar and was walking home when the attack happened.
She was grabbed near tents that were set up for the Rittenhouse Square Fine Arts Festival this weekend. It only stopped when someone heard what was happening.
"He fled because of a witness who interrupted the sexual assault and said something out loud," said Captain James Kearney, with the Special Victims Unit.
Action News learned that witness was with a security company hired by the arts festival to patrol the grounds through Sunday.
"All of our artists and our entire team was just devastated to hear what happened," said Sandra Sedmak Engel, an event organizer. "We're just so grateful that someone on our security team was here to help her."
On Friday night, sources said a suspect was in custody. He was picked up near 17th and Chestnut streets.
Police have identified the suspect as 42-year-old Lorinzo M. Clark. He was arrested and charged with rape, robbery and related offenses.
Police were able to identify Clark through video that was obtained during the investigation.
According to the Philadelphia District Attorney's Office, Clark's last known address is in Warren County, Pennsylvania, and he has an active bench warrant out of Erie County, Pennsylvania.
A mugshot of Clark was not immediately available.
Earlier in the day, pedestrians walked through the square, where tents are set up for an arts festival that is underway.
"I feel like it's a general surprise for this area. I always feel safe. We live a couple blocks up the street from here, and I've never felt threatened walking through the park," said Danielle Pett, who lives nearby.
"It's kind of scary. We live really close, a couple blocks away, and it's kind of sad people can't walk around safely at night- especially being a female," said Morgan Marshall, as she walked through the park.