A District police officer and three other men were convicted yesterday of two armed robberies of the Sears, Roebuck and Co. store in upper Northwest late last year. In one case, several of the robbers identified themselves as police officers responding to rumors about a pending robbery.

James T. Brown, who had been on the police force for one year, was convicted by a D.C. Superior Court jury of conspiracy and six armed offenses relating to the Nov. 18 and Nov. 29 robberies.

Brown was described by prosecutor Charles W. Cobb during the trial as one of two masterminds behind the robberies at the Sears store where he once worked as a security guard. Cobb told the jury that Brown and another former Sears security guard, David Eugene Burroughs, asked the other men to commit the robberies because the two of them might be recognized by store employes.

Burroughs also was convicted of six armed offenses and conspiracy in the trial before Judge Robert Shuker. The two men, who took the stand in their own defense, must be sentenced to a minimum of five years in prison because the armed offenses carry a mandatory prison term.

Cobb told the jury that Burroughs and Brown hired Bernard R. Ramsey to perform the Nov. 18 robbery, and described to Ramsey the routine followed by Sears security guards in collecting the cash from store registers. Ramsey, who allegedly also participated in the second robbery, was shot to death in December by Prince George's County police after a chase.

When security officers changed their routine after the Nov. 18 robbery, Cobb said three additional men were asked to help in the Nov. 29 robbery. It was during that robbery, Cobb said, that Ramsey, who was wearing Brown's police cap and badge, and the three other men entered the Sears store.

Cobb said the men identified themselves as police officers who were there to stake out the store because they heard another robbery was planned. When the men were asked for further identification, Ramsey pulled Brown's police revolver, Cobb said. Brown was arrested later that evening with three Sears money bags in his car containing about $1,500.

One of the three men, Brian K. Best, pleaded guilty before the trial to one count of armed robbery and testified against the others. The other men convicted of the Nov. 29 armed robbery were Kevin D. Taylor of 4523 Akron St., Temple Hills, and Richard L. Johnson, of 558 23rd St. NE.

The police department placed Brown on administrative leave after his arrest.