An off-duty city police officer critically wounded a man who was holding up a Hampden liquor store Friday night. Police spokesman Anthony Guglielmi said a customer at Hampden Liquors left the store as it was being robbed by two men about 8:20 p.m. The customer encountered an off-duty Northwestern District officer walking along the 3700 block of Falls Road. The officer went to the store and saw a 20-year-old man pointing a handgun at a clerk. Guglielmi said the officer, while on the street, "took a defensive position" and fired shots at the robber, wounding him in the upper torso. His accomplice fled the scene but was later caught on nearby Elm Avenue. Police recovered a second handgun in the 3600 block of Hickory Ave. Guglielmi, who said that no store employee was hurt, called the thwarted robbery "act of heroism." The suspect was taken to Sinai Hospital, where he underwent surgery and was listed in critical condition. Guglielmi said evidence and videotapes linked both men to two earlier retail store robberies. He said the first holdup, at 7:50 p.m., was in a shop the 1900 block of E. 31st St. The second was at a mini-mart in the 2000 block of Maryland Ave. about 8 p.m.