A Montgomery County patrolman was placed on administrative leave after the shooting death early yesterday of a 39-year-old Kensington man who allegedly had made a threatening gesture with a knife, police said.

Police said the shooting occurred at about 2 a.m. at a traffic light at the intersection of Wisconsin Avenue and East-West Highway when Bruce Jackson, an off-duty patrolman, approached the driver of a car who had reportedly been involved in an argument with another motorist.

Jackson and another off-duty officer told investigators that they had seen the driver, Jan Michael Moore of 3508 Perry Ave., leave his auto and approach the car behind him at a red light at the intersection of Bradley Lane and Wisconsin Avenue. Police spokesman Phil Caswell said Moore yelled at the motorist, returned to his car and drove north when the light changed.

Patrolman Jackson, who was driving his own car, approached Moore's auto and saw the driver holding a large knife, the police spokesman said. Police said Jackson re-identified himself before Moore attempted to leave the car with the knife.

Jackson stepped back and shot at Moore, Caswell said, then Moore drove north for three blocks, striking a parking meter, a parked car and a utility pole. Moore was pronounced dead at the scene and was found to have a bullet in his chest, the police spokesman said.

An autopsy has been scheduled to determine cause of death. Police said they were seeking witnesses, in addition to the second off-duty officer.

In police shootings, the officer involved is routinely placed on administrative leave with pay. Jackson is a nine-year veteran of the force. The last instance when a Montgomery County police officer killed a civilian occurred in September 1980, police said.