A 20-year-old Adelphi man was arrested yesterday and charged in one of a series of elevator robberies here after he was allegedly spotted atop an elevator in a building on upper Wisconsin Avenue.

The suspect was charged originally with unlawful entry in the Wisconsin Avenue incident, in which no robbery was reported. Last night he was also charged with a Feb. 4 elevator robbery at 1707 H St. NW.

In recent weeks there have been at least 10 cases in which passengers in elevators have been robbed by persons riding atop elevators in Montgomery County, Arlington and the District of Columbia.

Investigators have been hampered by the relative invisibility of the robbers to their victims. After being ordered to surrender their valuables, the hapless elevator riders have seen little more of the robbers than a hand reaching through the trap door in the roof to collect the loot.

Security and maintenance people were put on the lookout for suspicious activity, however, and when a building engineer working on an elevator at 5225 Wisconsin Ave. NW spotted two men atop an elevator in an adjoining shaft yesterday, he went to a telephone and called police.

After arriving at the building, police were directed to two men who were heading for a nearby bus stop. Hurrying to the bus stop, police reached it just in time to watch the two men board a crosstown bus and to see the bus pull away. The officers radioed a call for help.

Other officers, in cars, intercepted the bus at Connecticut Avenue and Legation Street NW, where the two men got off and began to run. The police ran after them.

Police said they arrested Anthony J. Reed, 20, of 7911 Riggs Rd. They said he was being questioned in connection with all the elevator holdups, but declined to reveal what led them to charge him in the Feb. 4 robbery.

Earlier yesterday one investigator said police might try to obtain a footprint from Reed for comparison with prints taken from tops of elevators in which robberies have occurred.

A second man is being sought in connection with yesterday's incident.