Two men robbed an armored truck in front of a downtown Washington bank yesterday and, while running from the scene, exchanged gunfire with one of the truck's guards in front of the National Museum of American Art as bystanders dived for cover, D.C. police reported.

Police said that one of the robbers might have been shot, but that no one else was injured in the gunfire. A man was arrested soon after the shootout, according to police, who said they were searching for two men, including a possible lookout.

Police said the robbery occurred about 2 p.m. as two guards with Federal Armored Express Inc. were delivering coins and currency to the Riggs National Bank branch at Ninth and F streets NW.

While the armed guards were rolling the money into the bank's F Street entrance on a small dolly, police said, two men with handguns confronted them on the sidewalk, and one of the men struck a guard in the face with a gun.

The robbers then grabbed a bag of coins and ran north on Ninth Street with the other guard in pursuit, police said.

"I heard all this screaming, then saw these guys run up the east sidewalk," said Roland Millard, an attendant at the PMI Parking lot in the 600 block of Ninth Street NW.

"Then this security guard with a gun runs up the street yelling, 'Where did they go,' and someone pointed around the corner" to the front of the National Museum of American Art on G Street.

"I got the hell out of the way, then I heard about two or three shots," Millard said.

A woman who operates a hot dog stand in front of the Martin Luther King Memorial Library, 901 G St. NW, said that about five people dived for cover behind her stand while the robbers and the guard exchanged gunfire.

"It was like a movie," said the hot dog vendor, who did not want to be named. "I thought they were fooling around until I saw the guy in the bank uniform."

The vendor said she huddled with a man, a woman and three boys behind the five-foot-long hot dog stand, which was about 100 feet behind the guard and in the line of fire.

"We were scared. All of us were scared back there," she said.

Police said that the robbers apparently ran in different directions after the shootout, and that one of them may have been wounded.

A short time later, police arrested a suspect near the intersection of Seventh and G streets and recovered "a portion" of the money, which consisted mostly of coins, according to Assistant Police Chief Isaac Fulwood Jr.

The suspect, identified as Horace Graydon Jr., 39, of 36 G St. SW, was charged with armed robbery, police said.

Police said that the other man was still at large last night, and that a third man, who police believe was acting as a lookout and who was possibly driving a getaway car, is also being sought. They said that one gun was found on the street near the armored truck.

The last armored-car robbery in the District occurred in May 1979, when a Federal Armored Express guard was shot in the shoulder and a bag containing $106,000 was stolen from him as he walked through the lobby of the Health, Education and Welfare Department headquarters at 200 Independence Ave. SW.