A 69-year-old man, robbed at gunpoint yesterday while strolling on the Mall with his wife, doggedly pursued a suspect for about a dozen downtown blocks in a noontime foot chase that ended in the headquarters of the FBI.

Alerted by the arrival and hurried explanations of the victim, guards at the building went to a men's room and arrested a 22-year-old man who police said had arrived moments earler, claming an urgent need to use the facility.

The suspect was identified as Weldon O'B. Hayes, a recently discharged Marine who gave his address as 339 Ridge St. SE. Police said Hayes was charged with armed robbery. The victim was identified by police as William Huf, described by police as a jogging enthusiast.

Det. Louis Robinson of the U.S. Park Police, who investigate the noontime incident, provided the following account:

Huf, who had recently undergone a number of operations on his mouth, had com eto the Mall with his wife to visit the museums there. They were at 12th Street when a man approached.

Producing a nickle-plated revolver, the man demanded money.

"I'm sorry," Mrs. Huf replied, and told the man she had none. The man demanded money from Huf, who handed over all that he was carrying - about $80. Then the man ran off down the Mall toward 7th Street.

Huf decided to give chase. His wife screamed. While her husband ran after the suspect, she hurried to alert some joggers nearby, who telephoned police.

Meanwhile, the foot race was on. It went east to 7th street, north to Pennsylavnia Avenue, and west to the FBI building in the 900 block of the Avenue.

Bursting into the 9th Street entrance, a man told the guards on duty there, "I've got to go to the bathroom real quick, where is it?"

He was directed down the hall and told to sign in. Without signing, the man ran to the restroom.

Shortly afterward, Huf came in the same entrance, he told of the robbery and described his quarry.

Hurrying to the restroom, the guards called on its occupant to come out, hands up.

When he did, they said, they found that he had taken off one wind-breaker to reveal another underneath. They said he had a nickel-plated revolver in his helt and $99 in cash in his pocket.

Hayes, the suspect, faces a court hearing today.

"Of all the buildings," observed Huf, a retired civilian employee of the Navy Department, "it's ironic he had to run into the FBI building . . ."