An 18-year-old man was shot and critically wounded yesterday by a D.C. police sergeant in Northwest Washington during a struggle in which the man allegedly reached for a pistol, D.C. police reported.

The wounded man, Joseph Porter of 30 T St. NW, was taken to the Washington Hospital Center's MedStar unit, where he was in critical condition last night with a gunshot wound in the right thigh.

Dr. Mark Buchly, head of MedStar, said Porter's femoral artery -- one of the largest in the body -- had been severed by the bullet. "Is he going to make it? Yes," Buchly said. However, Buchly added, "He lost a hell of a lot of blood."

Police said that Porter has been charged with assault on a police officer while armed.

Details of the shooting, which police said occurred shortly after 5 p.m. near North Capitol and O streets NW, were sketchy last night.

Police officials said Sgt. Manuel G. Rivera of the 1st Police District, who was in uniform and on routine patrol in a marked police cruiser, was checking for illegal drug transactions in an alley that runs between O Street and Hanover Place NW when he spotted two persons there. Officials said the two began running when they saw Rivera, who followed them in his cruiser to the back yard of a building in the 1300 block of North Capitol Street, which runs parallel to the alley.

As Rivera got out of his car and approached the pair, officials said, he noticed that one of them had what appeared to be a handgun in his waistband. When the sergeant attempted to arrest him, a struggle began and, according to one police official, "there came a point where the suspect reached into his waistband to draw the gun."

Rivera then drew his service revolver and fired an undetermined number of shots, striking the man once in the leg, officials said. They said the man did not fire.

The wounded man ran about a block south to North Capitol Street and New York Avenue, where he collapsed and was apprehended by other officers from the 1st District.

The second person Rivera chased in the alley was taken into custody for questioning, a police spokesman said. Police refused to comment late last night on whether the handgun Rivera reportedly saw was recovered, but it had not been found as of 8 p.m., an official said. Another official said that no drugs were confiscated in the incident.

Rivera, 33 and a 10-year-veteran of the force, will continue with his duties as a patrol sergeant while the shooting is investigated by the homicide branch, a police spokesman said.