As students and tourists milled about Harvard Square in the late-afternoon sunshine on Saturday, a Harvard police officer stopped a car for a traffic violation and was surprised to find inside it a loaded machine gun.

Officer Jimmy McCarthy stopped the Nissan 240sx around 4:40 p.m. near Au Bon Pain after he noticed it driving erratically down Holyoke St.

When he approached the car, McCarthy observed the three occupants passing around a duffle bag inside of which was the Lugar 9 millimeter machine gun, police said.

"[Officer McCarthy] drew down on them and immediately called for back-up," said Lt. John Stanton. "Then [all of the other patrol officers on duty] showed up, and there was a very large crowd of spectators there as well."

"The incident was handled quickly and professionally, and it was a one-in-a-million that no one got hurt," Stanton said.

The machine gun had a bullet in its chamber and nine more bullets in its magazine, according to police.

"It was ready to roll," Stanton said.

Harvard officers took into custody the three juveniles, whose names they did not release, and charged them with various counts of weapons possession.

Two of the juveniles were female and the driver of the car was male, police said.

The car and machine gun have been impounded at the 29 Garden St. Harvard police headquarters.

Stanton said police noticed bullet-hole inside the car and have linked the vehicle to a drive-by shooting in Texas.

"It was quite a way to start the shift," said Stanton, who had just come on duty as watch commander when the incident occurred. "Because [McCarthy] handled it by the numbers, by the book, no one was hurt."