Police say a savage beating reported Monday at SEPTA's Jefferson Station was a sexual assault committed by an unidentified man.

The incident took place about 1:20 p.m. Monday inside the Regional Rail hub, formerly known as Market East, on 10th Street near Filbert in Chinatown.

A passer-by notified Transit Police that an injured woman was lying on the station's floor claiming to have been attacked, said Jerri Williams, a spokeswoman for the transit authority.

When officers investigated, they found the bruised and bloodied woman collapsed on the concourse and took her to Thomas Jefferson University Hospital, where she remained Tuesday night.

The woman told detectives from police's Special Victims Unit that the man, believed to be about 25 years old, met her outside the station and led her down into the track area, police said.

He asked the victim for sex and then began to attack her before sexually assaulting her.

In her initial statement to detectives, the woman said she had approached the man on the street, enlisting his help in finding the correct train to take her home, a law-enforcement source said.

However, the woman later changed her story, and detectives are trying to determine what brought the two together, the source said.

Surveillance footage from the scene shows the pair entering Jefferson Station together, then walking down onto the train platform, the source said. At one point, the two ignore posted warning signs, open a gate and take a flight of stairs down into an off-limits area containing tools and other equipment near the train tracks.

The assault happened in that secluded section, away from the main platform, the source said.

After the assault, the suspect fled the station. The victim crawled back back onto the platform, where the Transit Police officers later found her.

The suspect is described as a man about 5-feet-10 inches with a thin build, police said. He wore a black coat with silver trim, a red-and-white scarf, a black knit hat, black jeans and grey sneakers during the incident.

Tipsters can call 215-685-3251.