An elderly woman was punched in the back of the head after getting on a Red Line train car at the Harvard Square station on Friday, Massachusetts Bay Transportation Authority police announced.

The assault occurred at rush hour, when the 71 year-old female stepped from the Harvard stop into a crowded car, according to an article tweeted by MBTA Transit Police. With all of the seats in the car taken, the victim saw another female, identified as 23-year-old Boston resident Jada Campbell, occupying one seat with her purse on the adjacent seat.

According to the MBTA Transit Police Department website, the victim “politely asked Campbell to hold her purse so the seat could free up and the victim could sit down.” Campbell refused, and when the victim handed her the purse and began to sit down, Campbell threatened to physically harm the victim. The elderly woman began to move away, fearing for her safety, when Campbell struck her in the back of the head with a closed fist. Campbell then began threatening witnesses who alerted transit police to the situation.

After a “struggle,” police arrested Campbell on charges of assault and battery on a person over 60, intimidation of a witness, and resisting arrest. She was then taken to transit police headquarters for booking.

MBTA Superintendent Richard Sullivan spoke out against the assault at the Harvard MBTA station.

“We are judged as a society by how we treat our most vulnerable citizens, our children and our elders,” Sullivan wrote in an emailed statement.

“This was a disgusting display of violence,” he wrote. “It is our sincere hope Campbell's arrest will serve notice, behavior such as this or any acts of violence on the MBTA will will not be tolerated.”

Sullivan also wrote the MBTA will work with the Middlesex County District Attorney’s office to “ensure Campbell is held accountable for her egregious conduct.”

The transit police website noted that all events remain allegations at this stage. “All defendants are presumed innocent until and unless proven guilty beyond a reasonable doubt,” the statement reads.

This is not the first incident on the Red Line this fall. In September, a 66-year-old man was struck and killed by a train at Porter Square station.

— Staff writer Jordan E. Virtue can be reached at jordan.virtue@thecrimson.com.