UPDATE: One arrest made, more expected.

Transit Police report they are looking for four people they say beat a man on a Red Line train early Monday after he asked them to leave his girlfriend alone.

According to police, the attack happened around 12:05 a.m. on Monday on an outbound train between Broadway and Andrew. The victim told police:

This group began harassing him and his girlfriend (unprovoked). When the victim requested they leave his girlfriend alone they physically beat and assaulted him. Victim sustained serious injury requiring surgery.

If any of them look familiar, contact detectives at 617-222-1050 or send an anonymous tip to 873873 (tips can also be sent via the anonymous feature on Transit Police's SeeSay app).