Mr. Omar’s first deployment was in the southern city of Dara’a, near Jordan, where he and his 350-strong unit were sent in March to help crack down on intensifying demonstrations. He said he had been ordered to arrest and shoot at dozens of protesters, including many young students, who had scrawled antigovernment graffiti on the walls of the town.

“The army needed everyone. It was very brutal,” he said. “But if there’s an officer of the Mukhabarat next to you,” he added, referring to the country’s feared security services, “you don’t have a choice but to shoot.”

Every soldier was armed with 60 bullets and given new ammunition each night, Mr. Omar said. His unit shot at the protesters from above a roof overlooking the mosque, killing at least six people and wounding dozens more. One of his fellow soldiers began to scream uncontrollably when he realized that his 18-year-old brother, demonstrating below on the street, had been shot. The soldier buried him two days later.

Shaken by what he had seen, Mr. Omar said, he was determined to defect. But before he could act, he was sent to Duma, northeast of Damascus, the capital, to work in a security unit interrogating detainees.

Mr. Omar said he had been asked to take notes during the interrogation of prisoners, some as young as 15 years old. He said demonstrators had been blindfolded and forced to strip to their underwear before their hands were tied behind their backs. Interrogations were conducted by four or five soldiers and officers in a dark, windowless room. He said the interrogating officer had ordered him to write down confessions naming protest leaders, confessions that detainees were then asked to finger stamp rather than sign, since their hands were bound.

To force confessions, Mr. Omar said, the soldiers tortured the detainees with electrified cattle prods, beat them or urinated on them. Some passed out. Others bled heavily. Many disappeared.

“The soldiers demanded to know why they had gone to the streets and who had paid them,” he recalled. “It was painful to watch. At the beginning I couldn’t sleep, but after a while, I got used to it. But I could not live with myself if I had remained.”