A 28-year-old Washington man was shot and seriously wounded last night by an assailant who apparently singled him out from among a number of stunned and frightened passengers riding on a Metro bus in Southwest Washington, authorities reported.

The armed man boarded a 7E route bus about 10:30 p.m. at 4th and M streets SW, near the beginning of its run to Silver Spring. He moved rapidly up the aisle, drew a gun and fired, according to Metro spokeswoman Beverly Silverberg.

Other passengers ducked for cover or threw up their arms to shield themselves. Several gave statements afterward to investigators.

Silverberg said the assailant may have known the victim, but the nature of their relationship was not immediately clear last night. D.C. police said they believed it likely the shooting followed an altercation.

A George Washington University Hospital spokeswoman identified the victim as Thomas Toomer of the District. No specific address was available.

Police said Toomer was shot once in his upper left abdomen. He was taken by ambulance to the George Washington University Hospital, where he was in serious condition early this morning after surgery, hospital authorities said.

The gunman fled from the scene, and no arrest had been reported as of early this morning.