A 24-year-old woman was slashed on the face by an unknown assailant just off the Schuylkill River Trail near Walnut Street in Center City on Thursday afternoon, authorities said.

The attack occurred about 2:55 p.m. while the victim was walking up the stairs from the trail to the Walnut Street Bridge, police said.

The woman and a female friend had been walking on the trail when they noticed they were being followed by an unidentified man, police said.

As the woman began to walk up the stairs, the man followed her and said, "You don't know anything."

He then slashed her on the forehead with a sharp object, police said.

The woman escaped her assailant and sought safety in a building on Walnut, police said. She was taken by medics to Penn Presbyterian Medical Center, where she was treated for the three-inch cut on her forehead.

City Councilman Kenyatta Johnson, who has been active in establishing a Schuylkill River Trail Watch, said the volunteer safety program would be accelerated with a launch this month.

"This incident is another painful reminder that we must immediately step up efforts to make the trail safe," Johnson said.

Police described the suspect as a black man, 20 to 25 years old, six feet tall, with an athletic build, and a goatee or beard.

Detectives were reviewing nearby surveillance video for images of the suspect.

The popular trail, which over the last year has suffered occasional spates of robberies and assaults, winds about 10 miles through Philadelphia.

Earlier this year, trail users were the victims of several armed robberies.

Last September, female runners reported being harassed by teens on bikes, who would run into them, touch them inappropriately, or shout at them.