Marvin Powell Rozier, 33, described himself in federal court here today as a reluctant, frightened 18-year-old in 1967 who was drawn innocently by a Prince George's County police informant into a convenience store robbery in which he was arrested and a companion shot and killed by waiting police.

But on cross-examination by attorneys for the police, who are being sued for $9 million for alleged civil rights violations, Rozier acknowledged he had been involved in at least one previous robbery.

He also said he was familiar with guns and lived in a tough neighborhood where youths attended parties armed with pistols and knives and "wouldn't have thought bad" of him "for doing a robbery."

Rozier's testimony came in the eighth week in a civil jury trial before U.S. District Court Judge Herbert F. Murray. Three former and present Prince George's County police officers, including the second-highest ranking official of the department today, are accused of orchestrating a series of five robberies and burglaries in 1967 and instructing informants to recruit participants to commit the crimes.

The three officers were members of a group of detectives known in police circles as the "death squad," according to Rozier and several other plaintiffs suing the officers.

Attorneys for the police and county officials have denied the allegations, claiming the informants reported planned robberies and burglaries to them and police routinely staked out the targeted stores.

Rozier, an auto mechanic in Adelphi who is now married with three children, testified about the first in the series of five incidents--the attempted holdup of a High's dairy store in Adelphi on June 8, 1967.

He said police informant Gregory Gibson--a high school chum whose informant role was unknown to Rozier--approached Rozier, attempting to persuade him over the course of two days to help him rob the High's store. Rozier said he reluctantly agreed. Gibson, he said, selected the store, the time and the date and provided a getaway car. Rozier said he drove to the store with Gibson and a second man, William Hunter Matthews, 18.

Rozier said he was afraid to go inside, so he gave a .32-caliber pistol he had brought with him to Matthews, who entered the store with Gibson. Moments later, police waiting inside fired at Matthews, killing him after he aimed a gun at the store clerk's head and ignored an order to halt, according to police accounts. Rozier was arrested outside the store and Gibson was allowed to escape. Rozier, a burly, bearded man who speaks in a deep, gravelly voice, said he was subsequently indicted for armed robbery but pleaded guilty to larceny and was given five years' probation.

Rozier is the third witness to testify about the High's holdup. Gibson testified that Det. Joseph D. Vasco Jr., now the Number Two man in the county police department, directed him to recruit participants for the holdup, selected the location and provided a car.

John R. Cicala, a retired policeman and self-acknowledged member of the "death squad," testified that the holdup was planned "from A to Z" by police and that Vasco instructed Gibson to recruit a "couple of patsies" for the job.