Timothy Joseph Buzbee, the 25-year-old land surveyor accused of five rapes in the Aspen Hill area of Montgomery County, went on trial yesterday on charges of robbing a Montgomery County woman after grabbing her on a dark, residential street in December 1980.

As the trial opened in county Circuit Court, Assistant State's Attorney Barry Hamilton told the jury the evidence would show that Buzbee was the attacker in a "brutal assault and robbery" that was stopped when two citizens came to the woman's rescue and chased her assailant. The man they chased was "too fast" and disappeared, but Buzbee was stopped a few minutes later in the vicinity, Hamilton said. He was not arrested at the time.

Buzbee's defense attorney, Reginald W. Bours III, told jurors that when Buzbee was stopped that night, Dec. 9, 1980, it was "a case of mistaken identity, pure and simple." Bours contended that prosecutors were trying the case now--28 months after the robbery occurred--when it's "a pretty cold trail (because) the incident's so old."

The opening of the robbery trial marked the beginning of a series of separate trials scheduled in the next three months in which Buzbee will face five other individual charges, all of which involve the so-called Aspen Hill rapes. Buzbee has pleaded not guilty to all of the crimes.

The investigation of the 1980 robbery case was renewed last fall after police had begun to look at Buzbee as a suspect in the Aspen Hill rapes.

In his opening statement, Hamilton said that the robbery victim, Dorothy Barr, was walking to her home in the Four Corners area of the county after a trip to a nearby shopping center when she heard "heavy footsteps running behind her . . . and she was grabbed from behind" and told to "Be quiet . . . I won't hurt you."

Hamilton said Barr struggled with her attacker and was helped by two men who were leaving a home at 208 St. Lawrence Dr., in front of which the attack occurred. Barr's attacker, according to Hamilton, was wearing some type of Army fatigue jacket and "coarse, work gloves."

The two men, Donald Nichols, who testified yesterday, and James Bowers, chased and lost the attacker on a path that cuts between St. Lawrence and Williamsburg drives.

Nichols, questioned by defense attorney John Monahan, testified yesterday that he "couldn't see the face" of the man he chased, does not recall his clothing and could only describe him as a man of average size.

A few minutes after the attack, as Bowers waited on Williamsburg Drive, Buzbee ran up and asked what was going on. "Mr. Buzbee, on this cold December evening, had on a short-sleeve shirt. . . " Hamilton told the jury.

Bowers detained Buzbee, who lived in Frederick, Md., but said he was jogging in the area and that he knew a family that lived nearby, according to Hamilton. Police were called, and later that night Barr identified Buzbee as her assailant, Hamilton asserted.

Defense attorney Bours, in his opening statement, told jurors that "based on the circumstances that night, no arrest was made." His client, Bours said, "just plain wasn't dressed like the person who attacked her that night."

Bours told the jury the prosecutor will "try to convince you it was very odd that Mr. Buzbee was in a short-sleeve shirt that December night." But, according to Bours, the temperature that night was 66 degrees.

Police officer Phillip Metz, who interviewed the victim, was killed on March 27, 1981, during a nighttime break-in at the W. Bell discount store in White Oak. Bours told jurors yesterday the defense lost a key witness with Metz's death because Metz decided not to arrest Buzbee.