The 30-second encounter in Brooklyn began with that most routine form of morning commerce, the breakfast pastry purchase. Almost instantly, the scene inside a Dunkin' Donuts shop on Empire Boulevard in Flatbush turned ugly.

The customer pulled a knife and lunged toward the clerk's face and, with his other hand, tried to reach into the cash register. A man in line, at first stunned, quickly intervened, grabbing the man in a sort of bear hug, only to be stabbed in the side as the man pulled away and fled.

The wounded man, an off-duty police officer who had stopped for coffee on his way to work early yesterday at the 71st Precinct station, was not seriously hurt. The police were able to identify the suspect, they said, because a witness saw the license plate number on his getaway car, his mother's red Kia Sportage. The suspect is wanted in two robberies at Dunkin' Donuts shops, including one last month at the same one on Empire Boulevard, the police said.

While yesterday's incident was otherwise unexceptional, the store's security cameras recorded it in its entirety from three angles. The tapes, which were given to the police by Dunkin' Donuts officials and provided to reporters by the department, afford an unusual view of a felony in progress, a jerky, fast-moving tableau that ends with the officer chasing the man out of the store and doubling over, apparently in pain.

The tapes begin with the young man, whom the police later identified as Shron Killings, approaching the shop counter clad in a Yankees cap, shorts and a white shirt. He puts a dollar down on the counter and looks over his shoulder several times toward the shop door. As the clerk turns away to reach for his pastry and to bag it, the man, his hands hidden from view by the counter's edge, appears to be unsheathing a knife or opening a folding knife. When the clerk turns back to him and opens the register, the man jumps on the counter, landing on his hip, and leans across and lunges at the clerk with the knife in one hand, unsuccessfully reaching for the till with the other.

The off-duty officer, Vincent A. Schiavarelli, 24, who had walked in unnoticed and was waiting to be served, grabs the man while he is on the countertop. Officer Schiavarelli wraps his arms around the man, who appears to stab Officer Schiavarelli and break free and run out of the store empty-handed. The officer, reaching for his left side, chases him out of the store.

Officer Schiavarelli, who the police said has made about 60 arrests in two years on the force, more than a dozen involving felonies, was to be held overnight for observation at Kings County Hospital Center, the police added. The wound was about two centimeters long, but doctors were unsure of its depth, said Paul J. Browne, the Police Department's deputy commissioner of public information.

Mr. Browne said the police believe Mr. Killings, of 451 Kingston Avenue in Brooklyn, who turns 22 today, got away with $300 in the May 17 robbery at the Empire Boulevard store, in which he had help from an accomplice, and $400 in a May 25 robbery at a Dunkin' Donuts on Utica Avenue. The police also said he was arrested twice on a charge of gun possession.

In the attack yesterday, the police said, Mr. Killings left his dollar on the counter.