DAMASCUS, Syria — Rafiq Lotof strode through the Syrian capital’s Old City, past his father’s shoe shop, past cubbyhole bars and antique shops, through streets that in normal times would buzz late into the evening with tourists and wealthy Damascus families. But on this recent night, the shops were shuttered, and Mr. Lotof’s errand was a wartime one.

At the entrance to a Shiite Muslim quarter, Mr. Lotof inspected a new checkpoint guarded by a baby-faced 18-year-old clutching a rifle nearly his height. Fresh from training in Iran, the teenager belonged to one of the growing neighborhood militias that Mr. Lotof is arming and organizing on behalf of the Syrian government — part of a nationwide effort to enlist more citizens in the fight against the rebels challenging President Bashar al-Assad.

After volunteering to defend a Shiite shrine south of Damascus, the young man, Hussein Beydoun, said he was flown with 500 other Syrian Shiites to Iran, where Revolutionary Guards trained them to use rifles, rocket-propelled grenades and mortars. Proudly looking on, his mentor, Mr. Lotof, said the heavier weapons might come into play if rebels ever tried to breach the Old City’s walls.

“If they come,” Mr. Lotof said, “they might do anything.”

Mr. Lotof, a son of the Old City, has returned after years in America for what he sees as a mission to defend its ancient streets, relatively unscathed by two years of war. This area of Damascus, inhabited since at least the third millennium B.C., is for many Syrians the heart of the country.