A 59-year-old Alexandria man suspected of robbing a downtown savings and loan association by threatening to explode what he called a tube of nitroglycerin, was trapped in a taxicab and wounded by District police in a midday shootout at 18th and K streets NW yesterday, police said.

Two police officers involved in the shooting were uninjured. Several dozen street vendors and pedestrians who were in the area at the time fled or took cover in office buildings or behind parked cars. No bystanders were injured, police said.

The wounded man, identified as Charles Lawrenson of the 1900 block of Marthas Road in Alexandria, was listed in stable condition last night at George Washington University Hospital with gunshot wounds to his lower abdomen and right arm, a hospital spokeswoman said.

Lawrenson has been charged with armed bank robbery and assaulting a police officer, police said.

The two officers who fired shots, identified as James R. Byrd and Alphonza McLean, were placed on administrative leave, which is standard procedure, pending an investigation of the incident, according to police officials. Byrd is a 15-year veteran of the force, and McLean an eight-year veteran.

Police said that about 11 a.m. a man entered the National Federal Savings and Loan Association branch at 1000 Connecticut Avenue NW and demanded money. The man, who witnesses said had gray hair and wore a dark suit jacket and tie, told a bank teller that a tube of liquid he was carrying contained the highly explosive chemical nitroglycerin. He did not display a gun, police said.

The tube was later recovered, and bomb technicians determined that the substance was not explosive, police said.

After receiving money from the teller, the man walked quickly out of the building and hailed a cab in the 1800 block of K Street NW, about one block away.

As he entered the cab, a bank employe and a security guard who had followed the man pointed him out to two officers in a police car who had responded to a silent alarm triggered during the robbery, police said.

A second police car arrived on the scene and police stopped the cab, which was in traffic, and ordered the passenger to get out. The cabdriver ran for cover, police said. A few seconds later, the passenger got out and fired a handgun at the officers, who got out of their squad cars and returned fire, police said.

Witnesses said nearby pedestrians started running and screaming at the sound of the gunfire, which lasted about five seconds.

"It was all very confusing. You could hear glass breaking and people screaming. There was a flurry of about 10 gunshots and people were hiding behind cars. After it was over, the suspect was lying in a pool of blood," said one witness, who asked not to be identified. He said the rear window of the cab was "pretty well blown away" by the bullets.