Police credit fast action by two concerned citizens and an off-duty police officer for catching a would-be robber. Detectives say a man tried to rob the Regions Bank downtown on Texas Street shortly after 11:30 this morning by handing a teller a note. But, before he got any money, he ran from the bank -- confronted a woman outside -- beat her up -- and snatched her purse. Two private citizens chased the man until he fell down at Market and Milam and held him there while an off-duty officer who was nearby arrested him.  38-year-old Johnny Jackson has now been booked into the Shreveport City Jail on charges of second-degree robbery and attempted first-degree robbery. Police think he had an accomplice and they're lookinng for him now.