A Framingham man was arrested on a warrant charging him with assault on a police officer after he was found loitering in front of the MBTA Chinatown Station early Tuesday. Robert O’Regan, 24, is facing charges of assault and battery on a police officer and disorderly conduct, Transit Police said in a statement. Officers arrived at the entrance of the station at about 12:38 a.m. Tuesday after reports of a group of people loitering. Officers approached to disperse them, Transit Police said, and O’Regan was discovered to have an outstanding warrant from Framingham District Court. He was then arrested.Catalina Gaitan can be reached at email@example.com. Follow her on Twitter at @catalina_gaitan.