An armed robbery outside the Rosslyn Metro stop last night set off a vigorous pursuit and search by citizens and police from three jurisdictions that involved a helicopter, included a breathless foot chase across Key Bridge and led to one arrest.

The episode, played out before the eyes of dozens of startled and fascinated onlookers who were spending a Friday evening out in Rosslyn or Georgetown, began about 10 p.m. when a woman leaving the subway station was accosted by two persons, at least one armed with a handgun.

The two took the woman's purse, and then fled on foot towards the Key Bridge, with three citizens, also on foot, in pursuit.

Near Rosslyn Circle and the approach to the bridge, the three citizens lost sight of the two suspects.

But two Arlington police officers in a patrol car spotted the fugitives as they raced onto the bridge walkway.

As the suspects reached the middle of the bridge, the pursuing officers saw them whirl about, and begin running back toward Rosslyn. Stopping their car on the bridge roadway, the officers got out, and ran after the fleeing pair.

Shortly afterward, according to Arlington County police, a 16-year old youth was arrested in the parking lot of the Marriott Key Bridge hotel, which overlooks the approaches to the bridge. The youth was charged with armed robbery, police said.

The second suspect eluded searchers, who by now included D.C. and U.S. Park Police. In an effort to find him, police combed the bridge approaches and also looked through the corridors of the hotel.

A Park Police helicopter swept up and down the Potomac River, above and below the bridge, beaming its spotlight on the shoreline where officers prowled on foot.

The search was called off after about an hour. The suspect was still at large late last night.