A doctor was convicted yesterday by a Montgomery County District Court judge of assaulting an antiabortion activist with a hypodermic syringe outside the physician's Gaithersburg medical clinic in January.

Leaders in the antiabortion movement said yesterday's was the third criminal conviction of a health care professional in assault cases brought by members of their movement nationally.

Dr. Alan J. Ross, coowner of the Uptown Women's Clinic in Gaithersburg, was jailed for an hour and a half yesterday after Judge Paul Weinstein found him guilty of assault and battery and openly carrying a deadly weapon with intent to injure. Ross was led away in handcuffs as two dozen antiabortion activists who attended the two-hour trial embraced each other in celebration of the doctor's conviction.

"Prolifers have the right of any citizen to freedom of speech and they should not be subject to assault and harassment," said David O'Steen, executive director of the National Right to Life Committee in Washington.

Ross said yesterday he was miffed by the conviction. "They are bothering me and other professional people," he said. "I respect the judge and I'm sorry he doesn't believe me or the facts. I feel awful. I've never been in trouble before with the law, convicted or in a cell."

Under Maryland law, assault and battery carries no mandatory sentence, leaving punishment to the judge's discretion. On the deadly weapon conviction, Ross faces a fine of up to $1,000 or three years in jail.

But Weinstein released Ross yesterday on personal bond and deferred sentencing until after a presentence investigation is completed on the doctor. That investigation will explore whether Ross' behavior was an isolated incident or part of a pattern, according to a spokesman for the Montgomery County state's attorney.

Three members of the Pro-Life Non-Violent Action Project, a Gaithersburg group that claims 1,000 members, filed assault charges last month against Ross stemming from a separate incident in front of Ross' home. Those charges are scheduled for trial in District Court later this month.

"When you take the law into your own hands you have to pay," Weinstein told Ross yesterday from the bench. "You had a no-trespassing sign posted -- all you had to do was pick up the phone and call the police. But you lost your cool."

John F. DeLozier, a 32-year-old engineer from Mount Airy who frequently counsels people outside the clinic against abortions, testified that on Jan. 15, Ross threatened to kill him in the parking area in front of the clinic.

Ross then punched and kicked him and stuck him in the buttocks with the needle of a syringe, saying it would would infect him with hepatitis, DeLozier said.

Ross denied the charges, testifying that after one of his patients cried and asked him to make DeLozier stop bothering her, he came out of the clinic still carrying a syringe he had just used to treat a patient.

"He [DeLozier] had some thought that I was injecting him but I was just pushing him away," Ross testified.

Weinstein said he found Ross' testimony "less than candid."

About a dozen similar assault cases against health care professionals are pending nationally and more will be filed, according to Joseph E. Scheidler, of the Pro-Life Action League, a Chicago-based group that claims 7,000 members nationally.

The other two assault convictions, according to antiabortion leaders, were the January 1982 conviction of a clinic counselor in Chicago for spraying Mace at two demonstrators, and the November 1983 conviction of a suburban St. Louis center director for attacking a demonstrator and knocking a movie camera out of the demonstrator's hands.