A man believed to be in his early 20s was shot in the head and critically wounded on the Washington Monument Grounds last night in an exchange of gunfire with U.S. Park Police officers investigating a complaint that a man was selling drugs to visiting schoolchildren.

Park Police Detective Thomas P. Moyer, 36, was wounded in the left elbow in the shootout, which occurred about 10 p.m. on the 15th Street side of the monument as hundreds of visitors waited a few yards away to ride the elevator to the monument's top.

Moyer was taken to the Washington Hospital Center, where he was listed in good condition last night. A second officer who was with him was also reported taken to the center for treatment of a leg wound not caused by gunfire.

A group of seventh-grade students from Roanoke, who were near the site of the shooting fled and ran shouting to take cover at a concession stand, according to witnesses.

According to the account of police officials, Detective Moyer and Officer Renee Jeters went to the 15th Street side of the Monument Grounds in response to a telephoned complaint about an attempt to sell drugs to a busload of students from South Carolina.

When the officers arrived, the officials said, they spotted a man in a parked automobile and approached him. As the officers drew near, authorities said, the man jumped from the auto and began firing.

Although Moyer was hit, both officers returned fire, the officials said, and the man was wounded in the arm and head.

The man was taken to George Washington University Hospital, where officials said early today that he was in "very critical" condition.

His identity was not immediately released pending identification of relatives, but hospital authorities said he was believed in his early 20s.

The extent of Officer Jeters' injury was not known immediately, but it was not believed serious.

According to the report of one witness, the man had followed the students' buses in his auto for several blocks from the downtown area to the Monument Grounds.

The vehicle apparently used by the wounded gunman was parked late last night near the souvenir stand on the 15th Street side of the Monument Grounds. There was a bullet hole in one door of the car and a rear window was shattered. The police officers' cruiser was parked behind it.

At the time the incident occurred, several hundred tourists were lined up around the base of the monument, which remains open until midnight in the tourist season.