An undercover D.C. policeman and a Southeast Washington man were seriously wounded yesterday in a brief gun battle that erupted at Minnesota Avenue and Nicholson Street SE after the officer stopped the man outside a rooming house to question him, investigators said.

The incident sent several persons ducking for cover. Police roadblocks disrupted traffic for more than an hour in the heavily traveled area where the incident took place.

Bill Jepsen of the police department said an investigation of the incident disclosed that the officer, who was on undercover duty, observed a drug transaction and apparently was seeking information from possible witnesses.

The wounded officer, Sgt. Dennis L. Boarman, 28, of the 7th Police District, was taken to Greater Southeast Community Hospital with a cut hand and a gunshot wound in his upper body, police said. The suspect, Willie Gene Leonard, 49, was admitted to D.C. General Hospital with several bullet wounds in the chest. Both men were reported in serious but stable condition. No other injuries were reported.

Leonard was charged with assault with intent to kill, police said.

Meanwhile, within hours of the shooting, about 35 officers across town in the 3rd Police District held a public roll call at 14th and U streets NW to honor the memory of another policeman, Arthur P. Snyder, who was shot to death a year ago yesterday in a drug-related incident. Snyder's alleged assailant, Bruce Wazon Griffith, a small-time drug dealer, was shot and killed by police three days later.

Yesterday's shooting indicent began about 10:45 a.m., police said, when Boarman, an eight-year member of the force, stopped his unmarked car to question Leonard in the front yard of a two-story row house at 2328 Nicholson St. SE.

A police official said Leonard apparently stabbed Boarman when the officer tried to question him and Boarman opened fire. Leonard then entered the house and emerged with a handgun and wounded Boarman, who meanwhile had radioed for help.

A 24-year-old woman who asked not to be identified said she was an eyewitness to part of the incident. She said she heard loud voices and looked out a window to see Boarman and Leonardf struggling with each other in front of the house.

"I saw him [Boardman] stike him several times," the woman said. "I didn't see a knife but I did see the officer's hand bleeding. [Boarman] stepped back on the sidewalk and fired between four and five times." The woman said she did not see Leonard stike the officer.

She said Leonard then entered the house, came out armed and began firing at the officer. Instead of firing back, she said, the sergeant retreated about 75 yards, then collapsed moments before fellow officers arrived.It was unclear why Boarman did not fire additional rounds, but there was speculation that he was out of ammunition. Leonard, meanwhile, collapsed in the yard of the house where the incident began.

Leonard's landlord and other residents of the neighborhood said Leonard had lived there several years, and was known throughout the neighborhood for his daily regimen of exercising outdoors.

At yesterday's police ceremony at 14th and U streets, Deputy Police Chief Rodwell M. Catoe called for five minutes of silence by the assembled officers.

"We've learned a lot," Catoe said. "At that time [Snyder's death] we just flooded the streets with officers. Today, you won't see as many uniformed men . . . but you also woin't see the drug problem at the same level. And we have done more than shift it from one place to the next. We have made some progress in elimination of those responsible for the problem."