A Southeast Washington man who was sued by Mayor Marion Barry and his wife, Effi, for injuries Mrs. Barry said she received in a 1984 traffic accident has filed a $3 million countersuit in which he claims he was wrongfully beaten by a plainclothes policeman who was driving Mrs. Barry's car.

George J. Nelson, a disabled laborer who lives on $323 a month in Social Security benefits, said the police officer, Vertice Gore, failed to identify himself before trying to grab the keys from the ignition switch of Nelson's car to keep him from leaving the scene of the accident.

". . . [Nelson] was insulted, cursed, threatened, punched, hit, beaten and otherwise physically assaulted both with hands and with dangerous weapons," according to the countersuit filed this week in D.C. Superior Court.

Nelson, who is seeking $2 million in compensatory damages and $1 million in punitive damages from Gore, the mayor and Mrs. Barry, contends he suffered emotional distress, public humiliation and permanent disfigurement. He maintains that Barry and his wife were "vicariously liable" for Gore's "wrongful conduct," which was "outside the scope of his employment" with the D.C. police department.

Barry and a lawyer for the mayor and Gore declined to comment yesterday on the allegations contained in the countersuit.

In a suit filed earlier this month, the Barrys contended that Mrs. Barry received serious neck and back injuries on Feb. 7, 1984, when the unmarked police car she was in was struck from behind by Nelson on New York Avenue near South Dakota Avenue NE. Mrs. Barry, who has suffered from a spinal disk ailment, was wearing a neck brace at the time.

Barry and his wife are seeking a total of $2 million from Nelson and his companion, Rosa Samuel, who owns the car Nelson was driving but was not present during the accident. Gore and his wife Joyce, also plaintiffs in the Barrys' suit, are seeking a total of $1.5 million in damages from Nelson and Samuel.

Nelson was arrested at the scene and charged with drunk driving and assaulting a police officer in his scuffle with Gore. Nelson pleaded guilty last Aug. 28 in Superior Court to the drunk driving charge, according to his attorney, George A. Teitelbaum, but the assault charge was dropped.

Nelson, 63, who suffers from emphysema, and Samuel, a 53-year-old cook, said recently that there was no way they could raise the money if they lose the case.

Donald J. Chaikin, a lawyer representing the Barrys and the Gores, yesterday declined to comment on the countersuit. He did say, however, that his clients are seeking the money to make an example of Nelson for driving while intoxicated.

"We wanted to show that when drunks hit people, not only would they be criminally responsible, but civilly responsible," Chaikin said. "If the defendant had not been drunk, the lawsuit would not have been for as much as it was."

Nelson said that he was driving no more than 10 mph in congested traffic when his car hit the police car. Police said that when Gore tried to prevent Nelson from leaving the scene before police arrived, he was assaulted with a screwdriver and a fight ensued.

Nelson denies that he brandished a screwdriver. He blames Gore for failing to identify himself as a police officer and for using unnecessary and excessive force in attempting to subdue and arrest him. Nelson stated he was taken to D.C. General Hospital for treatment of "severe" injuries following the accident.