Purse-snatchers, beware. Damsels in distress are fighting back; their Dudley Do-Rights are running that extra mile.

That, in essence, is the message one juvenile purse-snatcher carried away last week after he yanked a dark brown leather shoulder bag containing $90 off the arm of Laura Thrower, 29, as she was walking home to her Georgetown apartment at 8:20 on a cool Monday night.

He pulled. She pulled back. He pulled. SHe pulled back. He pulled and took off down Q Street with the purse, toward Rock Creek park. She kicked off maroon high heel pumps and gave chase in her stockings.

"HELP! HELP! HELP!" she shouted. "THIEF! THIEF! I'M GOING TO GET YOU!"

Thrower is a bright, dimunitive (5'2") woman normally given to shyness with strangers, say friends. She said later, "I was furious and I was in shape. I'd just started jogging the week before. I was ready to run."

The daughter of former IRS Commissioner H. Randolph Thrower, she had worked late in her office downtown at Partners of the Americans, where she serves as cultural program coordinator. She was writing letters and polishing a speech she was to give the next day in Syracuse, N.Y. She stopped at a Florida Avenue deli, bought some sesame dessert bread, realized she needed more money for her trip and withdrew $40 from Riggs National Bank's 24-hour teller at Connecticut and T streets, to add to the $50 she was already carrying.

"I was daydreaming about dancing," she recalled, a fancy that accompanied her to the corner of 27th and Q, where she noticed a shadow as she was walking west down Q Street. "My first reaction was that someone I knew was having fun with me, but that didn't last long."

The shadow was quiet. She took a step; the shadow took a step. In cadence. Silent.

Suddenly, the shadow turned into a man, grabbed the purse and ran. "He took off toward Rock Creek Park. I was running and screaming, 'Police! Police! Help! Someone, help!' I've had dreams like that, where you don't know whether you are going to scream or freeze. I've always had a fear I wouldn't be able to muster a scream. But I didn't have any trouble."

Commotion pierced the drawing rooms of Georgetown. People abandoned TV sets; dinners got cold. Concerned citizens, in fact, poured out of apartment buildings, and a crowd of perhaps 20 persons, she guessed later, formed at the scene of the crime, where she had dropped a briefcase.

By that time, though. Thrower was well down the road yelling, "HELP! HELP! HELP! I'M GOING TO GET YOU, THIEF!"

Thereupon, the damsel met her Do-Right. An Iranian student, 27-year-old Kurush Kimyabayki, was waiting for a friend in the lobby of an apartment at 2500 Q St. He heard the distress, a cry for help he felt pierce his "heart and soul."

Kurush told his friend, I'll be right with you," and disappeared out the door.

He saw a petite woman with short brown hair galloping toward a tressle the neighborhood has come to call Buffalo Bridge, after its Wild West bas-relief. He caught up with her. The blood had drained from her face. "What happened?"

"I just got mugged. That guy grabbed my purse! He's gone down the hill."

"Which way?" asked Kurush.

"That way!" And he was off. He saw a figure scramble up a hill on the other side of Rock Creek Parkway and disappear into the bush. He stalked cautiously, fearing the suspect might be carrying a knife or a gun, lost him for a moment and, finally, stumbled on a teen-age boy who appeared to stand 5-feet-4 and weigh 120 pounds. He was hiding behind a tree.

"Come back here, kid!" bellowed Kurush, who, upon seeing that the huntee was unarmed and smaller than the hunter, suddenly swelled with courage. The quarry, of course, was not about to come hither, and sprinted for yonder Georgetown. He tried to melt into the crowd. Kurush flagged a police car and climbed in.

He spotted the suspect down the street. An officer motioned the boy into the cruiser and everyone drove back to the site of the tug-of-war. "Is this the kid?" asked the officer.

"Yep, that's him," said Thrower. Her purse was found beside the hiding tree; the $90 was later retrieved from his pants and tennis shoes. He spent the night in the Receiving Home for Children. The juvenile judge set the trial date on the felony count for April 26 and released the boy to his uncle. Both parents were said to be dead. During the snatch, a dry cleaning hanger cut Thrower's hand slightly, but a doctor in the crowd dabbed at the wound. Otherwise, she was unhurt.

Friends told Kurush, the son of a district railroad commissioner in Iran, that he was crazy to take such a risk.

No, he told them, he was not so crazy, really. He had cemented many friendships this way. "I have met some marvelous people by helping out, people who are still my friends. I don't come from a wealthy family; I don't have much money. But life is filled with pleasure if you've got friends. I've gotten my reward," said Kurush, who is trying to transfer from the University of Kentucky to an architecture school hereabouts.

Once, he returned a 4-year-old girl who had been lost in the streets to distraught parents. In college back home, he was forever turning in lost wallets and car keys. And back in high school in Tehran, he modestly confessed, he received a medal for bravery; Kurush had rescued a student who got locked in a bathroom.

The Kurushes of the world ar forever stopping to fix the flat tires of strangers; and the strangers are forever calling them crazy.

Nor could Thrower fathom such behavior. She was equally shocked at her own.

As for Do-Right's reward, well, the damsel plans to take him to lunch.